If IMC holders don't practise flying in IMC, they won't be able to use it successfully to get out of trouble. Instrument flying degrades more quickly than most other skills; you need to keep in practice.
Bookworm,
I guess I realised I was answering a slightly different question. I just felt it needed saying. People beat themselves up over not flying, when the fact that they didn't feel like challenging themselves is, as you say, a perfectly good reason for not flying. I think it's something we sometimes forget in the "must push yourself" culture of the keen GA pilot.