Doubtless someone will disagree with me, but personally I don't see anything wrong with using continuous carb heat as a prophylactic measure in conditions likely to lead to carb icing. The air going into the engine will be unfiltered, but that shouldn't matter in flight (as opposed to ground operations). Engine efficiency will be slightly reduced, but that too shouldn't matter very much in cruise (I would not recommend leaving carb heat hot while on finals, in case a go-around is necessary).
See
here for the PPRUNE thread on the use of carb heat. For other discussions, see
here and
here.