The difference though is that verification through phone numbers relies on money that you've already spent, which is a lot more reasonable for the majority of users. People would still be unhappy if they had to pay 5 bucks if they didn't own a phone.
Its about increasing the cost of spam right? For some money is better than using your phone number, for others, they can continue to use the phone. Neither is exclusive of the other.
I know for some they would definitely choose the $5 over the phone for very good privacy reasons.
Less unhappy than the current situation where they can't use the site at all.
Also there's precedent of sites doing this. MetaFilter charges $5 to create an account. The bitcoin wiki used to require a fee (in bitcoin)[1]. Something Awful forums have a fee.
