I know he has a tendency to snatch kills but it doesnt change the benefits he brings to an adc. Basically all the reasons that top didnt get ganked when teemo was there is now sent to bot. The same is true for enemy bot and more. The supp cant wonder randomly for fear of the dark lords shrooms to ward. He can screw w/ the enemy adc with the blinds as well. wait for them to come in to last hit and pop em. you win a trade and deny farm. and while your torturing this poor team no one dares interfere for fear of being caught in a minefield and quite possibly killed, unsure if they could even escape sometimes if your smart about your shroom placement. All this while your adc is having a delightful minion feast, w/ not a care for the outside map.
To sum it up. Anything teemo fails to provide that a traditional support could is made up for with the absolutely disgusting zoning he forces on the enemy team. plus if you even get a few kills and assists one or two items is enough to make this possible and basically claim some jg along with a lane.