Ask an Aran thread.

it totally kills your combo though, which prevents you from being able to use any of your other skills. I maxed it on my first test server aran but I haven't on any since then because of how little I used it the first time.

It kills your combo if you don't max it (120 seconds between having to recast?)
Drain is extremely useful for the poor arans out there, and the other arans that don't want to use pots constantly.
Drain is perfect at 15 at the moment, but that's because i'm only at c-2. You could take 4 points out of rush (making it 8 or 9 instead of 10 mobs) if you actually wanted to max booster.