There should be some. They can be like a galactic "think tank" and becoming a Borg would be like training to being joined to a Symbiont on Trill, where the honor of becoming Borg is a very special privilege reserved only for those who prove they would have something to offer the collective. That way, the Borg can be as powerful as logic suggests they really should be given the technology and applications of that technology that is canon in the Star Trek universe, (continuity has a lot to answer for) but to make them not be generic "deus ex machina" aliens (which they probably could be) they would have no interest in dealing with societies as primitive as the Alpha Quadrant races. The Prime Directive works both ways. There may even be some Voth Borg.