It's all trick training, if you ask me.... getting the ultimate prey animal to allow a predator to ride him, sidepassing, spins, flying changes, jumping...
I agree with Sonny. It depends how any training is done whether it's good, bad or otherwise. I really like clicker training philosophy, tho I rarely actually clicker train these days. Negative reinforcement(removal of pressure) is a great teacher and fine, so long as it's not overdone and the 'pressure' you're using isn't too strong.
Specifically teaching tricks for the sake of fun is great I reckon, for the attitude of both horse & handler. It teaches the horse that he will be rewarded for following your direction, to have fun with his handler & not be 'micromanaged'. It teaches timing and the importance of positive reinforcement to the handler and generally keeps them in a good frame of mind - hard to get grumpy with a horse just because he can't kick a ball very well!
Obviously some tricks can cause potential probs, such as rearing, pawing, teaching the horse to use his mouth, haute ecole.... as with teaching anything else, we need to be aware of the possible lessons that we may be teaching the horse and make sure we teach them what & how we want in a way that will avoid or minimise the possibility for 'bad' behaviours.