There are a few other differences between an IMC and an IR:
1. Hold entry/flying the hold.
2. IR's tend to be on a twin so you have asymmetric approaches.
3. Engine failure during a go around/asymmetric go around.
4. Airways.
5. Non precision approaches/Precision Approaches to much tighter limits.
6. SID's and STAR's
There really is no comparison between the two. And for those who say an IR is used multicrew - my IR is for single pilot commercial operations. However I can only use it commercially when I have 700 hours so most people do a multicrew IR when they do a type rating on a multicrew aircraft.
After saying all of that, I would say that an IMC is a useful rating, but as others have said currency and airmanship are the main issues. The IMC gives you a bit more help in a marginal "go/no-go" situation but it is not a "mini-IR" and should not be treated as such.