I think you can practice your game in freerolls, you just have to adhere to proper strategy...which usually involves playing the opposite of what the table is doing.
I don't play in many freerolls, but when I do, I work on certain points of my game: stealing blinds from weaker players, trapping tight-aggressives, etc.
I would never play freerolls to improve my game on a widescale basis, I just use them as cheap (free) practice to work on some of the smaller aspects of my game.
The only way to avoid the maniac syndrome is to not play in the freerolls.
I'm not sure I agree with this. Obviously you will not have to deal with maniacs if you don't play in the freerolls. You don't have to get "maniac sydrome" as you called it in freerolls...this is called TILT. Sure, you may get sucked-out on, but continually going in with the best hand and occasionally taking a bad beat and handling it well are good confidence boosters, and improve your play.
Once you start calling all ins with Q-10 offsuit, then you know you have to take a break, walk around the block, and regroup.