I did the IR(r)/IMC during my hour building and it was invaluable for the following reasons:
1. A new challenge and a different sort of flying.
2. Improves your flying/planning/decision making abilities.
3. When you have it you can hour build when the weather would have previously prevented you from doing so.
4. Great preparation for the IR. Most of the course you would have already done - It will be about practise and perfecting techniques rather than discovering it all for the first time!
5. It was fun.