[userbinator]

it's to cut down on spam from users unwilling/unable to set up tens or hundreds of phone numbers, which I imagine is the majority of spammers.

If anything I think it's the opposite --- dedicated spammers have shown they can farm resources like accounts of various types, so phone numbers aren't out of their reach. It's the casual users who don't want to give away their phone numbers or setup a throwaway one which will be turned away.

[hirsin]

Cost proof doesn't cut in for those users - it's typically only put in on the Nth new signup within X hours from an IP address.