We considered this, but one issue is that unlocking all Champions in practice will drive more people into practice mode and split our player base. More players in MM means shorter queues and better matches. Practice mode is not the best way to experience our game, and we shouldn't make that the superior mode.
"Level 10-15 No Leavers allowed"
and you get people all = level, with no leaves.
Best 60 min back and fourth gameplay experience in LoL hands down...its actually sad more people don't take advantage of this but alas most don't because there is no stat tracking and after 4 games no IP/EXP.
Which is depressing because it makes the game 100x better.