The important thing with Blade of the Serpent, is that ooze regains mana SO FAST with it. For instance, at rank 4 ooze, you get 37.5 + 101.25*targets
So, even if you just stand next to one enemy, turning it on with level 4 ooze gives you 140 mps. You are probably spitting, attacking, etc them too, and very likely in a group of creeps.
Point is, you can generate thousands of mana in 10 seconds, often using as many skills as you can, and still coming out with full mana. Games past level 4 really show the strength of this favor item.