Actually under JAA this isn't theoretically the case, its written into JAR that a MP/IR should also renew a SP/IR but unfortunately most member states didnt adopt this policy initially. The was talk a while back of the UK CAA changing but I dont think it ever happend. I guess we will have to wait and see what EASA brings now.
I agree with portsharbourflyer, its daft and it keeps the costs of GA rising. I know many commercial pilots who would have returned to light aircraft (after all the flight training loans have been paid off) but are not prepaid to go through the time and expense of a full CAA IR flight test after the 7 years have elapsed on the rating.