ICBM nails it, I think. It's there to retain people who've cost you £6M+ to train, and have spent goodness knows how many £XM training since, and whose replacements would not only cost £6M+ to train but would take 5+ years to be ready; and that's without putting a price on the interim years of operational experience. Therefore, if they PVR, they haven't been retained, so take it off them immediately. That seems like a "fair" system. But if you've retained them, don't look for any excuse to screw them out of it!
The side effect of that would mean capable officers wouldn't be afraid of doing what whenurhappy stated, and looking at out-of-branch roles. I and many of my peers would love to be able to broaden ourselves as officers by taking a couple of years outside of flying to do something totally different, but there's simply no way I'd take the risk when it'd involve a loss of almost 25% of my salary. That and the chances of getting back to flying are relatively slim, so it'd be a permanent loss; yet they've still retained me, and I still have all those skills, and all that knowledge....