notwithstanding all the other caveats mentioned here, wouldn't hashing on the client side make it possible to salt the hash so that different sites generate a different hash, thus making it unlikely that the hash can be reused even if the actual password is the same? the salt could even include a time component making the hash expire after a time.

this obviously does not eliminate the need for other security measures, so it's possibly more a question of "is it worth it?"