The problem with this is that key generation is free, so being a well-behaved unknown bot is the same as being an unidentified bot, which means that you go in the block/captcha/throttle bucket.

It is only useful for whitelisting bots, not for banning bad ones, as bad ones can rotate keys.

Whitelisting clients by identity is the death of the open web, and means that nobody will ever be able to compete with capital on even footing.