One of those brainless decisions which pointlessly causes trouble for clients. Account separation at IB is now a formality as all accounts belonging to a single person/entity are accessed using a single user/pass/keycard combo, through a single TWS window. An IB rep on the phone actually tried to tell me that this doesn't pose any kind of additional security risk to my money. Now I have to worry about a previously-nonexistent fat finger risk, as it's inevitable that one day I'll route a huge order to the wrong account or otherwise get mixed up. Aside from that, TWS is a horrendously slow and buggy piece of crap when it comes to flipping between different accounts. Portfolio and account info takes forever to load, often the wrong information or figures are displayed, and the currently selected account seems to often switch at random (walk away from the computer, come back and a different set of numbers is showing). First time I can say I'm really disappointed with IB. But I guess they're pretty much the only option if you don't want your accounts to be looted by Jon Corzine.
Also, you can set up TWS pages so that they ALWAYS and ONLY show a specific account. Just go through the "add page / quote monitor" routine and you 'll notice how.