I have to echo the poster who wrote that often it is not a POS who is encroaching on your space. On a recent flight I was seated next to a very nice lady who did not require an extender but would have definitely been spilling into my seat, were we not in the exit row with the fixed armrest panels. Being aware of her size, she arranged herself carefully so as not to displace me and it was a pleasant enough trip. However, on the earlier leg of my journey, I was seated next to a wiry young man who, once airbourne, decided he was entitled to the entire row and proceeded to stretch and sprawl and did everything but get in my lap. Horrible. I agree that extremely large PAX should be encouraged to purchase two seats; however, I also think common sense and good manners should win out, no matter what, and there should be some way to gently (or not so gently) steer space invaders of any size back into their proper territory.