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.
I could see that... "Good Borg"... they assimilate voluntarily, fix all your social problems, and supply you with an endless stream of "happiness" serum.
Instead of being "assimilated by" the collective, people are "integrated into" the collective, which is why they have to be so discerning as to who becomes Borg. If you had a collective of the Hawkings, Einsteins, Newtons, Darwins, etc. then you wont want the Paris Hiltons of the galaxy in your collective dragging you down.
The Mirror Universe Borg appear in the Mirror Universe novel sdroW htoB fo tsroW ehT by Greg Cox in the volume Glass Empires (2007). Like the other Mirror Universe species, there's no major difference between the the MU Borg and RU Borg -- except that Mirror Universe Borg are ruled by the Borg King instead of the Borg Queen, and their ships are diamond-shaped rather than cube-shaped.
So what you're saying is that Obama is a mirror universe Borg thinking he is doing good but actually is not?