Having done both the IMC and JAR ME/IR ratings, I'm surprised to learn that the approach minima for IMC-only rated PPL's are advisory. I was taught that the DH on an ILS for an IMC rated PPL was never less than 500 ft above rwy threshold height, and never below 600 ft below rwy threshold height for an NDB approach.
Personally, I found that having an IMC rating before doing the ME/IR was not as useful as I thought it would be. Although I was familiar with approach plates and procedures, the step up to flying IFR in a twin was a big one. Also, the IR is done to commercial standards, with big emphasis on managing the flight, check lists etc. You are very very busy flying instruments in a light twin to commercial standards, and the IMC rating did little to prepare me for this.
I agree though that doing an IMC would be useful for your hours-building cross-country flying, and is a good way to become more familiar with weather and flying approaches in IMC.