As a husband to a wife with rheumatoid arthritis, I can tell you that Southwest's preboard policy works out really well for us. Wife and I get to board early, guaranteeing her a seat that works well for her frail hip, and we get off first so that we don't have to spend any more time than necessary in the seat. While she is cursed with arthritis, we still appreciate the opportunity to sit together in a seat that's as comfortable as possible, and we return the favor to passengers by stowing our carryons under my seat so that others can have more bin space. (And yes, my wife has all the paperwork to prove she's disabled. She may not need a wheelchair, but she's still disabled.)
We're not all bad people who deserve to sit in the back of the plane, you know. ;-)