Vivienne makes it pretty clear that while she is doing to help with the recent chaos, she is mainly doing it to gain all she can wth the Inquisition. It's succsess, its gaining of prestige, its becoming a dominant power in Thedas, etc. I appreciated that forwardness. Which I appreciated, but still dislike enough to not recruit her for.
No matter what you do, she has everything to gain and nothing to loose (she still appears in the ending of Trespasser, and it is implied she still manages the Circles.). The Inquisition could only move her up, and she is skilled enough to make sure it never drags her down. Refusing to let her join will only force her to seek other avenues for her political maneuvering.
Similarly, while you too have things to gain if she joins you (namely a companion and a few table operations) you essentially loose nothing of note if you choose not to recruit her. As far as gameplay is concerned, all you really loose when you don't recruit her is a knight enchanter.