Yea I agree doing all around stuff I think is much better for their mind and body! Lol. They use different muscles in different disciplines and keeps them in shape. They don't get sour and it keeps things interesting. Yea they will be better at some things then others but that doesn't mean you shouldn't do it. Like everyone said common sense. Don't push your horse to try to something it physically can't do. Have realistic expectations on what they can do and so on.
I love doing everything my gelding! Halter to Speed, reining, pleasure, trail, jumping dressage, everything!! He loves it is sound and we are even going to mess around with some cows soon! Hehe!