i've been in the freewheel on the fixed side camp for several years. used it in races, mountain bike trails, loaded touring, no problems. everytime a shop employee told me that it was "wrong" i'd give a hearty guffaw and go order my parts online (which realistically i'd probably have done anyway).
now i am a part time shop employee, and i've seen three stripped fixed/fixed hubs that had freewheels on them. to be fair, they were stock on giant boweries (out of a probable twenty our shop has sold) and i don't think they were formula or dimension so maybe it's a hub quality issue.