(“. . . he will stop paying attention.”)
If you are going to compete at the FEI levels, you want to ride on the best possible footing available. And in general when you school, the fewer distractions you have, the easier it is to progress. Slipping and falling on your head would be one such distraction! There are various other times where uneven or deep footing is more than a nuisance: teaching your horse to make medium trot is a good example.
That said, within reason it’s worthwhile to expose your horse to less than perfect conditions. I don’t mean where he’s likely to get a stone bruise or pull a tendon or have his feet slide out from under him. But if the surface he works on is always “too perfect,” he will stop paying attention to how he places his legs. In short, you want him to develop his coping skills to deal with moderately sloping ground, uneven places, and especially with puddles. These are all things that are part of real life, and you do your horse a disservice if you don’t make him familiar with them.
My own early riding included a lot of fox hunting. Picture plunging through a hock deep bog or skidding down an ice covered blacktop road with the sparks flying off your horse’s borium-shod heels. You would not much find me doing those things today, but it does put our “encase them in gauze and bubble wrap” philosophy in perspective.
Without endangering your horse, the more things he’s familiar with, the more he can help you which, in turn, frees you to concentrate on important matters—the next fence or the next movement, for instance—and not be waylaid by details which should be his concern, not yours.