Grasp is the way to go with Teemo

You are right, but: a) Teemo is almost always in combat because poison b) it is easy for him to proc it twice if your enemy tries to trade you c) it is super-easy for Teemo to autoattack his lane opponent every 4 seconds for a bit of sustain and damage, that adds up

Many Teemos go Hextech these days, so they get sustain quickly, but for those that don't it seems strong, Teemo's big problem is that if you deal damage to him, it will stick to him.

I don't do it myself though, I like Hextech Teemo better, so don't believe what I say too easily.