I've almost needed a seat extender on a few RJ flights and I'm well under 200 pounds (but pregnant). Totally depends on the aircraft and seatbelt and way the passenger is built. I'd bet there are some fit, solid, 250 pound men who can't fit in some of those seats without an extender either, and I'd much want them in an exit row over a 120 pound weakling.

But let's face it, the chance of needing to get out that door is so slim it's not really worth worrying about.