Proving Grounds is super good for practicing mechanical team fight stuff. I've said this before, but I learned how to play AD carry pretty much solely on ARAM and Dominion. The laning phase of Summoner's Rift is pretty useless for teaching you anything other than CS'ing. I see a lot of less experienced players falling to pieces after laning phase because they suck at positioning and being able to really utilize their champion mechanically. Dominion and ARAM on the other hand are basically constant team fighting practice.
"Stranded in this spooky town." LoL IGN: Matt Is Legion, A Mole Sitting