What purpose would this mechanic serve? How would it make the game fun?
What's to stop the player from telling the game that they want to autofarm 9 million rats and instantly hit the level cap?
Using bad mechanics to cover up other bad mechanics is never a good idea. In this case it would be better to remove the 20 bat wing quest entirely.
I have thought about this kind of autoplay feature before, also. I'm not sure I would call this feature a "bad mechanic" per se, I'm just not sure it would really be needed in a game that gave the player alternatives to bat farming. A task is so trivial that a player can do it in a completely mindless way shouldn't still be giving them a useful reward.
But I don't want too quick to dismiss this idea. Xecutor, can you please tell us more about how you thought that it might be used? Are you thinking that it is impossible to completely remove situations like the one you described from the game, so there should be a way to make them more manageable?
Oh, and if there was a time limit, that might discourage players from abusing this feature to reach nine million xp by killing bats. Or, just if bats stopped giving xp after 500 had been killed, or whatever. I realize that just leads to more questions related to balance, though.
Incidentally, I think that I am now agreeing with zasvid regarding level grinding, at least as far as this discussion goes.