My experience has been similar to RustyC's; I've had very few empty middles in stretch. My luck has been better when choosing the exit row stretch seats instead of up front, YMMV. I made a comment about this to a gate agent at RSW recently and he said it was because they move the people requiring a wheelchair into the stretch seating up front to get them on and off easier. I kind of doubt that's a Frontier policy, just the gate agents at veerrrrry wheelchair-heavy RSW adapting to move things along quickly with tight turnaround times... but who knows.