I will admit to this also. If I decide to carry a 1911 I am pretty diligent about the spare ammo, M&P (typically) not so much. I say typically because when people are rioting or going batshit for TP or otherwise being squirrely I get a little more squirrely, or is that spider sense?...
And another thing about being more standardized is I have plenty of magazines that fit all three sizes of 9mm M&P I would be carrying and there is a spare in a carrier with a simple quick clip in both vehicles.
And yeah it is important for more than just ammo capacity, but I am just an aging suburban schumk going about my life. If I actually meet my demise from running out of ammo when I stopped for beer you can make fun of me, I won't care if I am dead.