The only problem I see if you want to really advance in a certain discipline. For example, if I wanted to seriously show my horse in WP, I probably wouldn't make a habit of barrel racing. If you want to be serious with cows, barrel racing might not be such a bad thing. If I had a reining horse, pleasure or trail would compliment nicely. It also has to do with your horses attitude. A friend of mine almost bought a horse who could literally do it all. Reining was her main, and they occasionally did barrels and cows with her, which she was ok at. She just wasn't the type to let it "go to her head".