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.
I have heard this many times and it still confused me. It somewhat seems like the aviation equivelent of the car Airbag. To my simplistic mind, surely after doing an IMC you are safe to fly in IMC (assuming currency, etc, etc) or by definition you haven't completed the IMC
Regarding the comparison of IMC to IR Isn't an IR a completely different bird in most respects. I imagine (i.e. I don't know) that the lower DH and the tolerances required to fly in controlled airspace warrent a significant chunk of the extra hours involved.