I'm on board with the account-switcher functionality. I think it's even on the big feature wishlist.

Bad users are going to create new accounts to circumvent bans no matter what the rules are. Honest users are going to link them together because they want the convenience. So a multi-account limitation only punishes honest users.

How I'll probably implement it (simplest/easiest for me to build) is that you have N number of slots on your edit-profile page. You put the username/password combo for each account you want to link together and click "Save". Now your current account and each of the linked accounts will have some quick-login dropdown or something.