I'll give you the real answer to the question of whether this is a good idea:
How many transplants (as in: not from Ohio) live in Cincinnati?
The higher the ratio, the better an idea it is. Sports is a religion and you buy into it as a kid. You grow up with your "favourite teams" and you never change them, even if you have no logical reason to be a fan of that team anymore.
If you have a ton of transplants who love living in Cincinnati but couldn't bear to emotionally part with their childhood favourite teams, the MLS team will become their "Cincinnati team" if you market it right.
Year one was boffo for United here
.....mind you the Thrashers had good attendance in year 1 also and we saw how that turned out. But then AGAIN, that team was a tirefire and United isn't. My theory is that a lot of Atlanta-loving transplants who never emotionally abandoned their beloved Blackhawks (or whoever) have no qualms adopting a MLS team from the city because they were never fans of their hometown MLS team in the first place.