Most people who haven't been close to someone with SCI think that "being in a wheelchair" is just some sort of inconvenience, and they can perfectly well sit in a line just as long as someone with no problems.
In reality, a person whose spine is injured severely enough to be unable to walk, very often has serious problems with lower body functions (not to mention overall functions, like blood pressure, temperature control, etc.)
These include things like constant urinary tract infections, incontinence, need for catheterization, or truly bizarre syndromes, like experiencing painful complete body spasms when their bladder is full.
These aren't rare symptoms, but are very frequent among people with spinal cord injury - and there are many other conditions that also cause bowel and bladder problems. Often, but not always, a person with these conditions will be using a wheelchair, walker, or cane.
So when you're in line in the public restroom, and somebody wheels in, for God's sake,
offer them the next turn at the big stall. (And if somebody asks for the next turn, be polite and let them ahead, because every problem isn't visible. )
Here's some useful forums on SCI bowel and bladder issues:
http://www.apparelyzed.com/forums/forum/42-spinal-cord-injury-bladder-bowel/
http://sci.rutgers.edu/forum/forumdisplay.php?f=15