Then they wouldn't have to use as many multiple accounts, which would mean less money given to SE assuming they pay for accounts legitimately. If they have X gil on server A, and y gil on server B, but they need more gil on B, they could easily move it from A to B by creating an alt and sending it mail. No need to go through another account registration. Sure they'd still have multiple accounts just in the off chance that some of them were banned, but it would make passing money and items between servers a lot easier if it was all shared.
My first thought when I saw the first post about no shared bank for alts was that RMT would probably love to have such a thing.