Hi craigber,
true, even if we would talk about a grown system in maintainance phase that runs for years at your customer and you know it's stable, if you don't move to the newest SP for that reason, you might also never move to any newer version at all. In retrospective it's the reason there still some DOS apps.
Mostly a system is unstable when it's new and needs to be cured from it's children's illnesses and when it becomes old. Not updating to new versions or only SPs is leading a system to get old, as it's environment will grow and update. There are bugs that can byte you, even if they did not so far for a long time.
Bye, Olaf.