I figure that large customers are bound to fill the stretch seating rows, whereas regular rows are more likely to have an empty middle. True?
My experience is that the stretch seats often fly empty. I guess people looking for a cheap ticket aren't willing to spend the extra money.
I was a Frontier Elite for a while and I was always able to upgrade at the gate and often sat in a row alone. Of course that was almost 2 years ago. I liked Frontier, I only stopped flying them because they dropped my most frequent route.