I think tread does improve traction in the less than perfect world of urban cycling.
Like, around corners with gravel. There's a LOT of coarse blacktop out there. Or, when you cut a lawn around campus or wherever for some non VC reason. Not big honking knobbies though. But fat tires are more grippy for sure.
Conti Town and Country or TT have inverse tread and a lot of solid angular rubber extending to the sidewalls for cornering grip.
And, hey, even if it's partially psychological, I'll take the advantages of a little tread on a big tire. Continental Town and Country 26x2.1s are some of the best urban 26" tires around. They are on a lot of police bikes, if you haven't seen them in your LBS.