Jack Of All Trades... Master of None
Western is not usually a primary focus of SIM games, and is vastly misunderstood. The competitive western horse is just as complex as the english horse.
"The english horse!?" you say. "My horse is an show jumper! He's not an 'english horse'!"
Yup, the english horse. My horse is a barrel racer. He's not a 'western horse'. But there are some things you have to tolerate.
So, what is the point I'm trying to prove? You wouldn't show a warmblood that's bred, trained, and built for show jumping in, say, park classes, would you? There's no way in the world that the warmblood could have the movement, build, or even attitude that it would require. So, if you do keep reading, you'll see why a quarter horse (or any western breed), cannot possibly cross western dicsiplines.
We'll start with every western discipline, and explain the various factors that would keep that particular animal from cross entering into other western disciplines.
Western Pleasure

The western pleasure horse. Where to start. The typical western pleasure 'build'. I'm going to base this on the 'true' all around quarter horse, which is the only all around that is realistic. The 'western pleasure, hunter under saddle, western riding, trail, showmanship' type horse.
The western pleasure horse typically has a longer back (but still very balanced), smooth, flowing look. They're bred to naturally keep their headsets even with their poll, and typically have nice shouldar/pastern angles for a smooth ride. When breeding a pleasure horse, looks are a serious factor. Pleasure horses are typically relaxed horses.
They're bred for movement, attitude, and conformation. They have to have a smooth, slow-er movement, with cadenced gaits. Put in a better way (and quoting the AQHA), a good pleasure horse has a free-flowing stride of reasonable length in keeping with his conformation. Ideally, he should have a balanced, flowing motion. The quality of movement and consistency of gaits is a major consideration. He should carry his head and neck in a relaxed, natural position, with his poll level with the level of his withers. He should not be nosed out (carrying his head farther out than 120degrees) which would show resistance on the part of the horse. His head should be level with his nose slightly in front of the vertical. He should be shown on a loose rein. The horse should give the appearance of being a pleasure to ride.
Pleasure horses typically don't have the conformation required for agile turns, quick roll backs or working cattle, as most cutting horses do carry their hocks farther under them than a typical pleasure horse. They also lack the speed required for any roping or gaming events.

Barrel Racing/Pole Bending
Barrel horses, on the other hand, are not bred for conformation, or attitude, or really anything but speed and agility (Ok, so even the breeding for agility doesn't happen at times). Most are off the track horses, and they're typically hot, well muscled horses with shorter backs than that of pleasure horses, and have a completely different 'look'. They don't have the smoothness to their movement required of pleasure horses, the proper head carriage, or the attitude it requires. I have had more experience with barrel horses than I've had with pleasure horses, and you might be saying "Well, I had a barrel horse that was calm and could ride on a loose rein, etc." Well congrats, but you're one in a million. Ty Murray even said the only thing more dangerous at the National Finals Rodeo than bull riding is being the guy that leads the barrel horses to the arena. At right is a picture of a horse with "ideal" barrel horse conformation.
Most typical game horses compete in barrel racing and pole bending. However, the NFR quality barrel horses compete solely in barrel racing. At times, barrel horses get too hot to run a pole pattern, and become straight barrel horses, or don't have the agility required of pole bending (the sorrel mare below, Spirit Woman, was strictly a barrel horse, for both reasons).
Even if you did have a calm barrel horse, I wouldn't recommend showing it in a high quality pleasure class. They usually have a faster cadence to their movement, lack anything 'flowing' about it, higher set headsets, more of a performance build, and I've never met a barrel horse that can work on that loose of a rein.
Below are two more pictures, (of two of my wonderful mares, I might add) One is a conformation shot of Lil' Encinita, and another is a picture of Spirit Woman running a pattern. Neither of which look like they could be pleasure material (and neither of which are. SW, although a highly trained amazing barrel horse, can't even back up, which is required in all western pleasure classes.)
Roping
If there are any two disciplines that can merge well out of all the ones listed, it's speed events (barrels/poles) and roping, although that is rare. The bay mare above, Lil' Encinita, was both a roping horse and a barrel horse, but she was world class at neither. She could have been, at either discipline, if focused in that discipline. I stopped roping with her and began running barrels/poles only, and she qualified for NBHA World. Jack of all trades, master of none.
The roping horse requires two things that most barrel horses lack: a 'whoa', and cow sense. Roping horse conformation is more of the 'foundation' quarter horse. You want a stocky, well muscled, wide horse that can put the halt on a 500 lb steer running at full speed, while carrying a (on average) 200lb rider.
You'll often find roping horses crossed on barrel horses, and then trained for roping only. Mainly, due to the fact that roping horses have to be fast. If you have a horse that can't catch your calf, you won't do well no matter how well a roper you are. Even roping horses are more specialized than you might believe. Just about any roping horse can do breakaway roping, but most calf roping horses are strictly calf roping horses, and most heading horses are striclty heading horses, etc.
Typically, you don't want a cutting or working cow horse to be a roping horse, because you don't want a horse in either of these disciplines to learn to 'chase' or 'track' a calf, as it might confuse them with their cutting (or working cow horse, for that matter). If you had a cutting horse chase a calf around the arena, it's time to find a new event. Note; the horse (below right) is only a two year old.
BTW, a roping horse also makes one heckuva team penning horse. You don't typically want to use a cutting/working cow horse for team penning for the same reason you don't typically want to use them for roping.
Cuttng, Reining, Working Cow Horse
Another combination that is seen is the cutting/reining/working cow horse. It's not extremely common, but you will find horses talented enough to do all three events. They may not win the world in all three, but they can compete succesfully. There are downfalls to that combination, but it can work. (There's a reason that the competition for horses to compete in reining, working cow horse, cutting and steer roping is called The Worlds Greatest Horseman and Horse competition).
These horses obviously have to be built for agility, quickness, and they have to have cow in them. I've touched on these events all through this page, so I'm not going to go too in depth. You might be quick to jump on the fact that these horses, like western pleasure horses, have to work on a loose rein, so they should be able to work in both disciplines. Not so. Reining, Cutting and Working Cow Horses typically have quicker gaits, carry their head too high, stop too quickly with their hind end too far under them, and back fairly fast. They also tend to do rollbacks quickly versus the pleasure horses' pivot. They're built more compactly, have shorter backs for more agile movement, and tend to carry their hocks farther underneath them than that of a pleasure horse, for quick stops and movements while riding.
Conclusion
Now that I've bored you with all the disciplines (and I didn't even go into steer wrestling!) I must quickly point out a few more factors that would limit a horse from competing in every event.
When you breed a horse for an event, you usually focus your bloodlines so you get a superior horse for that particular event, whether your breeding for speed, looks, movement, mind, agility or cow.
Training a horse for any of the above listed events is not an easy thing. If you have a world champion calibre reining horse, you have to remember how much time went into his reining training, and how focused he had to be on that.
