Take the TB21 or the Saratoga for example. Very capable aircraft complete with airways nav avionics. Yet put a couple of clouds in the sky below freezing level and your grounded!
Talking to a couple of the charter pilots at my school, I get the impression that despite the Seneca's certification for moderate icing, it is not a pleasant experience and is generally avoided as if it wasn't capable of dealing with that type of icing. As far as light aircraft are concerned, I remain to be convinced that de-ice equipment is much practical use for serious situations such as moderate icing. I suspect much simpler and more robust systems like prop and window heat provide the get out of jail without the notion that you can get into jail first.