Shaco's thrown dagger also gets an AP bonus, but it gets an AD bonus as well. The point is AD Shaco barely impacts team fights even when doing crazy hallucinate damage. Getting rid of his AD drops him to total uselessness. It's a scrub build because it's used by bad players with the intent of killing bad players. They set traps in bushes and k/d whore. Then when team fights start they lose.
Teemo is not tanky dps, but he's currently best itemized that way because of his incredible squishiness and short range.
Lately, I've taken to describing some champs as having secondary roles. Champions that fulfill a normal team role, but not champs you'd want as the only one filling that role. It's basically like off tanks, but with other roles. For instance Lux and Morgana are good examples of secondary casters. They do a lot of damage, but are more likely to get kill stolen from than other casters so they fill a secondary rather than carry caster role. In that vein I'd say Teemo is a secondary ranged AD. He doesn't snowball, so you might as well give him some defense items to compensate for the fact that people tend to go for him first.
