I will probably never be in a situation where I need to use my pistol, but I'll be darned happy to have it if that situation ever does occur.
I carry a spare magazine for the same reason. If I ever do need to use my pistol, I'll probably never need a reload. But if I do, I'll be darned happy to have that spare magazine.
And as other posters have stated, it's more for a magazine-related failure rather than needing the extra ammo.
We can play "what if" all day, but what if you're in a scuffle with someone over your pistol, and you finally gain control - but during the fight your magazine was ejected? What if you're tackled while your pistol is still holstered, and you hit the ground hard enough that the magazine release is hit. You manage to recover and draw your pistol, only to have the magazine drop out?
Yep, in such situations, you'd be hard-pressed to grab your spare magazine and reload, but you'd be even *harder* pressed to do so if you don't carry a spare...
Of course, a BUG is even better...