I believe that repeated bumping can lead to damage of the tumbler pins and surround, since you're having to impart enough energy into them to force them to jump against their spring in order to make the procedure work.

Wikipedia notes that hardened steel tumblers are more susceptible to this mode of attack due to them being able to take more force without becoming deformed (which seems plausible, although I don't have any authoritative source)

Bumping can leave distinctive grooves inside the barrel and on the pins that a normal key wouldn't. They may consider these traces to be destructive even though the lock remains operational.