A good question.
The main scenario I can imagine where one might wish to keep a valid IMCR (and, by implication, a valid UK/JAA PPL and a valid CAA medical) is where one feels one can't keep up with the FAA currency requirements of 6 approaches in the past 6 months. Then, if your FAA IFR currency lapses, you can continue IFR in the UK on the IMCR privileges.
However, an aircraft owner should be able to easily maintain the 6/6 currency, but someone who is renting, and/or short of money, perhaps cannot. And very few non-owners will be flying under an FAA IR.
A rather more theoretical scenario is where you really believe that the UK Govt will succeed in their proposed prohibition on long term parking of N-reg planes in the UK. Then you would need to revert to the IMCR to have any IFR privileges at all. Or if EASA manages to do the same; however they have lately been indicating they would prefer the carrot route rather than the DfT's stick route.
I was planning to let the 150-quid UK medical lapse. I will keep paying for the JAA PPL every 5 years. It's easy enough to arrange a ride every 24 months which, in one flight, gives you the FAA BFR, the UK PPL renewal, the IMCR renewal all on one flight of an hour or so. Then, AIUI, if I ever need the UK stuff again, I will just pay up for the CAA medical.
"I can fly with greater than IMCR privilages in the UK in a G-reg aircraft if I have an FAA IR"
Sadly not. An FAA IR in a G-reg gives you IFR privileges in the UK but only in Class G, whereas the IMCR gives you IFR in D-G.
An FAA IR holder can purchase the IMCR from the CAA for just the cheque payment, but he also needs a valid UK or JAA PPL to attach that to...