Tapers are not a wet or dry tire, they work very well for both if the dirt is right for them.
As for why ribs are used in front on 2wd vehicles, it's simple. a 2wd buggy/truck places no drive or brake force on the front tires, only lateral loads so a front tire's job is much different from a rear. There is a bunch of voodoo science going on out there and sometimes a tire just works. People will come up with all kinds of theories why but nobody really knows.
Knob tires on the front can provide less aggressive/smoother steering on hard surfaces and the opposite on soft surfaces. It's a good idea to talk to the guys that race at your local track. Between all of them they prbably know what the best setup is.