Yes and No. Markets are not completely irrelevant. But they're not everything either. You have to actually deliever a market for it to really work. The idea of adding a Georgia Tech to deliever Atlanta or the U to deliever Miami is a fallacy. It just won't happen. When ESPN or Fox is paying money for broadcast rights, they do so in order to get programming people want to watch. They want ratings. If Georgia Tech really got everyone in Atlanta to tune in to their games, GT games would be rated high. But they're not.
As proven by the Big East, having schools in large markets does not mean huge money. Houston, Tulane, Memphis, Temple, SMU, Central Florida. These schools are all in large cities. Yet, when most of the schools were in C-USA, no one gave a crap. It's for those same reasons that conferences weren't falling over themselves to add Rutgers. Rutgers does not deliever a huge market share, despite being located in a very populous area.
The Big 10 model helps change that. They want cable providers like Verizon and Comcast to bundle their network to basic subscribers for high carriage fees. You get a big enough market and even if only some of them actually get the network, it's still a good chunk of change. And even if only a portion of the people that get the channel actually watch it, it does not matter (unless the carriage fees get to high and providers drop the network since not enough people care). You are collecting money from everyone that has the channel on their cable package.
This model only works with a Network. If a network is put on a majority of cable packages in a populous area, then the network and hence the conference make good money. But selling the individual rights to those schools may be problematic. It only works if people care about the teams. The Big 10 operates on the former. The Big 12 on the latter. For the Big 12, it only makes sense to add schools that people want to watch. Hence FSU and Clemson often get mentioned.
The idea that markets equal money is a huge over-simplification.