You're missing the obvious, Ken -- when other classes MUST run treads, or other competitors are already on treads. Those guys tear up the track (outlaw winged cage karts, etc.) Many tracks in the midwest and plains states run these cars together on the same night. Some even dictate treaded tires (as a cost saver - heh, "NOT") for some of their flat kart classes as well.
I would agree that slicks are faster if everyone else were on slicks too. But when the track "crumbs up" from others running treads, you about have to go to treads (or grooved slicks) too.