Speaking as someone who had an IMC rating - and used it - and then upgraded to the IR, I can confirm that the IR does go quite a bit further on airways procedures - both navigation and R/T - and equips the pilot with a lot more tools and techniques for flying to a higher degree of accuracy and repeatability in controlled airspace. But, if all you want to do is fly in the clouds, the IMC rating is quite sufficient.
I am certainly in the camp that believe the IMC rating can and should be used to fly actual IFR trips on a regular basis - it is not some form of emergency training. I can also tell you that if you've done a couple of hundred hours of actual IMC using your IMC rating, doing an IR is a doddle. There's still lots to learn, but keeping the aircraft pointing the right way up whilst doing all the other stuff isn't one of them.