It seems a lot more resource intensive to have a bot responding to SMS messages from many different phone numbers than more traditional ways of gaming the syste

It seems a lot more resource intensive to have a bot responding to SMS messages from many different phone numbers than more traditional ways of gaming the system. So unless becoming a user on your site is worth much much more than the average site, just the increased barrier, even if not totally secure, should be enough to keep the vast majority of bots away.