rjack22, welcome to Flytertalk! You didn't say where and when you're flying, which both can play a big role in whether there will be any empty seats. In my experience, mostly flying Fridays and Mondays in/out of ATL, there are never ever any empty seats, let alone empty EC seats.

Higher tier frequent fliers get EC seats for free. Granted most would not pick a middle seat if there is a choice, but if they are purchasing last minute (as many business travelers do), and there aren't a lot of seats left they'd take a middle in EC over a middle non-EC seat.