I'm impressed that you even know about this obscure feature - I can't even find it in the documentation! And I'm not even sure I remember how it works. (I vaguely recall some scheme to re-purpose the PolyShell swap key to tell ATE to switch between multiple client-side sessions, but can't remember the precise details, other than that Control-Tab is also supposed to do nearly the same thing. (I think the former skipped minimized windows while the latter doesn't.)
I guess I see no technical reason why ATE couldn't launch a new session on the Control-Tab (or other) swap key if there were no other sessions to swap to. But considering that it may take a few seconds for that to occur, there might be some user-experience downsides or complications to that happening in a context where it might be easy to do it accidentally.
Before deciding if this is really a good idea, you may want to review
Launching another ATE from ATE, in which another developer toyed with various approaches and seemed to have settled on a menu option to do so. But I'm not sure anyone even considered just making it an extension of the Control-Tab swap key.