In reply to Sarah_Clough:
Well I did read something... but I think you have to be realistic, and not just consider a theoretical case that just MIGHT occur. (Here I think the 'you're going to die mentality' quote that JKarran used somewhere is very apt). I can think of many hundreds of routes where the protection, whether it's pegs or nuts, is really marginal. If you didn't clip this or that because it wasn't what you wanted, you'd either fail on the route or hurt yourself. I really can't think of any insitu gear I'd choose not to clip. (Of course Al has a point if it makes the rope run badly...)
I know I'm wheeling out old photos, but look at this one, then read the comment I've posted against it.
http://www.ukclimbing.com/images/dbpage.php?id=106472
The RURP in question clearly won't hold a fall, but the really good small wires all around it stand a really good chance of lifting out if you don't clip it. I know you actually said you can think of plenty of good reasons 'for' clipping, I didn't miss that, but I just don't think anyone is going to not clip for your (actually not yours) reasons.
I also think that some of the old pegs are remarkably solid. Take the old peg on the Ochre Slab of Vector. Who would think that would hold a fall? Yet it must have held hundreds because as far as I know it was JB's original(?) and thus placed in 1960! If, of course, it's no longer there then ignore this para!