An excellent question. Like a lot of climbers, I was trained with the repeated admonition to "Never step on a rope! Especially with crampons!!" But the truth is, you have to step on the rope dead center to pierce it with a crampon tip. So while it's prudent to avoid stepping on a rope, particularly on hard, rocky surfaces, to step on one in the snow isn't necessarily fatal, even if you have pointy metal things coming out of your soles.
The thing to do is give it a very careful visual examination. If a crampon went through the sheath, you'll see the mark. If the sheath is intact, then the odds are very good that the rope is perfectly OK. Even fine for leading. And I would include glacier travel in the same league as leading, in terms of the potential load on a rope. After all, what if someone falls into a slot? That's going to load the rope nearly as much, or as much, as a leader fall. But generally, ropes are so tough these days that if it checks out visually, there shouldn't be any risk involved in using it.
My own feeling is that ropes and crampons have an almost magnetic attraction. I've spent many hours traveling roped over glaciers, but try as I might I even find myself tromping on the rope with alarming frequency. Just naturally klutzy, I guess.
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