Hehe. Definitely agree with the mask prom bit

No saving those.
I guess I've just been lucky. I've run across very, very few 'bad' EPROMs that I couldn't salvage. Ocassionally, you'll find some that won't blank all the way - I've had luck programming the whole thing with zeroes, then erasing it again. Sometimes you even have to do it twice. In my opinion, it's worth a shot, and costs nothing to try.
EPROMs that have had mechanical damage, on the other hand, corroded/broken pins, cracked housings, etc - those are just plain bad from the get-go and can't be salvaged. I've had a couple of the old-style ceramic with gold legs type chips where the legs have somehow rotted out, and they just fall off if you so much as touch them wrong. I think it's the same thing happening to them that happens to 70's era TI chips, the underlying metals corrode under the plating.
But, then again, I'm from a hobbyist/hardware hacker perspective, and it's always worth the extra effort to me to salvage whatever I can.
-Ian