Arguments based on TV markets are stuck in the same logic the ACC tried to use in 2003. Of course Blacksburg isn't a big TV market (nevermind that the biggest VT alumni base is DC) and Clemson doesn't bring anything to the table from a TV perspective. That's not what the SEC is after here. The SEC is selling its brand, not the appeal of any individual school(s).
The SEC already has national TV deals with CBS and ESPN, and those didn't come because Starkville, MS, Columbia, SC, and Fayetteville, AR, are media hotbeds. The SEC is selling the best football, the most intense football rivalries, and with the exception of Vandy, major state schools whose following is measured not in alumni but in numbers of people who currently live or who have ever lived in those states (plenty of Alabama and Tennessee fans in the Triangle who are from those states but didn't attend those schools).
Even for folks without allegiances to those schools, national audiences might not tune in to see Clemson play, but they'll tune in to see if the Gators can survive Death Valley if it's already on TV (which it will be). Plus, the SEC is pretty spread out, and Clemson to the SEC would mean that the shortest drive between two SEC schools would become Clemson to Athens. Clemson to Columbia and Clemson to Knoxville would also be in the top eight. So, the intense regional rivalry that is critical to the SEC brand is just waiting to happen.