Well, if you're in a city already, you might end up deciding to challenge the gym anyway, just on the basis of "why not, I'm here and I might as well". Also, the cities people are most likely to avoid because they're out of the way are Azalea, Cianwood, and Blackthorne, and two of those have some of the best rewards, the Fly HM from Cianwood and the sheer monetary reward from Blackthorne.