depends on the composition of your team really
ad/as teemo with the new move quick is a very good and safe damage dealer, and is very good at 1 man ganking
ap teemo has strong map awareness, and good single target nukes, but lategame against any good team oracles shuts down 75% of your damage
hybrid teemo (madreds / rageblade / survive items) lets you do strong damage with good harass with your spells
i've had some first pick teemos at 1700 elo rape face with AD/AS builds.
i suppose in your situation it's bad because he's last pick or near last pick, and you already have your team setup, but when you have first pick teemo and he tells you what he's building it's easy to form a good team and let him play the way he wants, and usually it'll work.
ad/as teemo with the new move quick is a very good and safe damage dealer, and is very good at 1 man ganking
ap teemo has strong map awareness, and good single target nukes, but lategame against any good team oracles shuts down 75% of your damage
hybrid teemo (madreds / rageblade / survive items) lets you do strong damage with good harass with your spells
i've had some first pick teemos at 1700 elo rape face with AD/AS builds.
i suppose in your situation it's bad because he's last pick or near last pick, and you already have your team setup, but when you have first pick teemo and he tells you what he's building it's easy to form a good team and let him play the way he wants, and usually it'll work.
Jet has been banned for trolling and blatant racism.

Uploaded the image since I had it done already for my Teemo guide, OMG SHH NOT A SPOILER!
In ranked, I go AP/Slight AD. I find that Teemo is strong in the map awareness field, and that it will force people to get an early oracles to attempt to take out your shrooms, putting them behind in gold. Teemo is just extremely powerful if played right. :]
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TL;DR: Why do people pick teemo in ranked?