I think that caves are far from overused in Tolkiens books. They are a great place for people to dwell in safety (Menegroth), Dwarves love caves, and basically all evil things like to dwell in caves to get away from the nasty sun.
It makes sense that people will encounter a cave now and then, live in one, or simply have no other alternative path than to go through one.
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Son of Isildur.
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