Simply put ... I would always prefer to find that at the end of a "situation" I still have rounds in my magazine. While reloading is always an option ... it's an option I would rather not be forced to exercise.
Comments about being "... attacked by a whole mechanized infantry division" and "... if you can't protect yourself with 8 shots ..." make for pithy and smart comments, but don't reflect the reality that I, as the intended victim, intend to dictate what I am armed with for my defense and the magazine capacity available to me.
While a multiple person home invasion is unlikely, having an AR15 with a reliable 30 round (or 40 round) magazine and facing only a single threat means that you are likely to have unfired ammo in your magazine at the end of the "situation".
"Remember, nobody ever complained about having carried too much ammo after a firefight!"
PS ... I fired my first M1 Garand as a Junior ROTC Cadet in 1961, trained with an M14 in 1967 and went to combat with an M16A1 in 1968 ... and I loved all three of them for different reasons.