Most will tell you they aren't since they won't be thick enough. However, most of my inlays don't go over .040" deep due to the limitations of the smaller cutters, so for things like that, they would have some use. I also do most of my inlay work when the cue is only .005" from finished size. The ones who say they won't work, I'm sure do it much earlier.
I don't know how thick the Ivory on Piano keys is as I've never used it. If it's at least .050, I don't see why you couldn't find some use for it. The silver sheets I buy are only .052" thick and that is used for any silver borders on inlays and all of my ringwork. I haven't clipped the edges on one yet that I can recall.