When I did the IMC I had it drummed into me by my F/I never to launch off into known IMC conditions - besides the problems already mentioned of calibration of instruments etc. the rating is simply not good enough to enable us to fly in IMC safely.
My understanding is that its useful to get you out of trouble and does improve your skill for VFR.
IMHO - if you want to fly routes in cloud then get an IR (if you can afford it!) and a de-iced/airways plane and fly every week to keep current!