[snappieT]

In fact, Frontier charges more for the middle seat. I assume this is because middle-seaters are likely to be a couple/family wanting to sit together and forced to pay the premium. No solo traveler would willingly purchase a middle seat, so you don't need to optimize for pricing the middle seats for them.

[sokoloff]

Seems this wouldn’t work very well. Pick the aisle and window; middle will always trade IME. If you need three across, book aisle, window, and a nearby aisle and you again have tradable seats.

Or book aisle and aisle for a couple.

[mikeash]

Maybe they’re just picking up a few bucks from people who aren’t this clever.