One benefit of not having vanilla registration is that you're more or less going to have all unique users. One could imagine gaming the system using multiple accounts.
Having 2 or 3 twitter accounts is not a big deal for his site. Creating 10000 vanilla accounts would be. Adding that kind of anti-spam methods is a lot of work and not easy to get right.
IP throtterling. now if they create the accounts on unique IP's then you have somebody who is overly dedicated. It is when the reward is less than the effort to cheat that you have a workable system of protection.
Beyond that there is no golden solution without costly verification process's.
Precisely. I'm not entirely sure how deep your site goes, but assuming it's like the actual stock market, one could potentially game the system by creating two separate accounts and making a series of investments with one, and by shorting those exact same investments on the other; One of the accounts is guaranteed to go up.
