Although I am a big fan of the front brake, I strongly urge using both brakes together on a slippery surface, because, as others have noted, it is almost impossible to recover from a front wheel skid (been there ... done that at low speed in a parking lot -- slightly painful lesson learned). I did manage to bring the same bike, Capo #1, a long wheelbased traditional road bike, to a safe stop when an errant piece of wood locked up my rear wheel. Last year a stick through the front spokes was fatal for a local gent.

I do concur regarding the importance of body positioning and brake handle modulation.