The change was brought about to ease the burden on crews, post 2003, and was fought by the SAR Force. I'd like to bet that there have been times when a Nimrod has been requested to prove topcover for a helicopter but refused because of timing issues since the change to 2 hours notice.
How many times has topcover actually been needed, not just provided? Never as far as I can remember. Although a nice comfort blanket for the SAR Force it is an inefficient use of resources.
A 2 hour standby does not require a serviceable a/c to sit on the line, it means an a/c needs to be available for a crew to launch within 2 hours. This allows more flexibilty to the fleet managers.