Yep, the only reason not to throw up another fort would be if (A) it would prevent you from recruiting more capitol-only units (if one of the 2 existing provinces is your capitol) or (B) if it's a lab on the front lines where you need to sit a bunch of mages who are there for defense, but you'd like to have them research instead of patrol.
Oh yeah, (C): If you haven't got the cash!
