User blog comment:TremendousDerp/Geminix - The Twin Diamond/@comment-5238542-20130605202841/@comment-8185594-20130606001116

25% of current health is a lot of health sacrificed; you'll likely only use it once or twice before you go back to base, or else you risk giving a free kill in lane. In the case of Gragas, he can use it whenever he feels like it since it literally has no cost; consequently, he'll have a greater net mana regeneration so long as he stays in lane atleast 2 minutes before he goes back. Besides, you should never max it first, and it's not optimal to max it second either. Even if you get health potions to mitigate the health reduction, considering you'll need atleast 1 health potion per cast to break even in terms of health (ex. level 4 HP: 701 * 0.25 = ~175, HP pots give 150 HP),  it needs to have a strong incentive to use it as otherwise you'll just fall behind for no substantial gain.

If you think of it backwards and reduce the cooldown of Gragas W from 25 to 15 and give significantly higher mana regeneration in return for reducing 1/4 of his current HP (which again, is a lot of health missing), I'm pretty sure that most Gragas players would pass the offer by, even with his mana problems. :P

tl;dr  It doesn't matter how much mana you get if you die for it or can't use it as often.