It would be interesting to hear reasons for their use on other lenses, beyond an abundance of caution.
I've taken my 70-300L down the beach a few times, to shoot surfers and things, the wind is always blowing a gale down there. Am I going to point my $1500 lens straight into the wind and have the front element sandblasted? Hell no, I point my B&W MRC filter into the wind, it only cost $60 or so, it has no measurable difference on IQ (as i tested on my 7D), and it's Multicoated so no extra flare in *normal* situations (i don't feel like testing it by pointing at the sun).
As to whether it's *needed* for weathersealing? Don't know, don't care. It can't make the sealing *worse*, and since it doesn't make the images worse either, then to me it's a no-brainer.
As for impact damage? Some people will claim that Hoods are better. They may be, it all depends on too many variables of height, floortype, angle, etc. Why not just use both? (And why not just don't drop it?) Hoods help a bit with less flare too. And if i dropped it i'd be more concerned about the IS elements rattling around and the mount breaking off the camera body than the outer elements smashing...