Zilean passive now store 12 EXPERIENCE at lv 18.

Zilean being at level 18 doesn't mean his allies are at level 18.

Imagine a mid lane Zilean who manages to give his jungler experience every time the jungler can gain a level and when Zilean has his passive off cooldown. Zilean likely will hit level 18 well before anyone else on his team will, and can still afford to give experience to his allies even if it has no use on himself.

Basically, Zilean will get to level 18 before anyone else on his team, then will help his teammates play catch-up with him to bring his allies up to his level faster.

Depends how fast it fills up. If it fills up every minute then its a good passive. If its every 5 minutes its terrible.