In today's civil society...
It is difficult to imagine a crediatable secenario where you, as a non-LEO, would need more than the ammo in your carry gun and your onboard reloads (spare mag, speedloader, etc..) before the situation is resolved.
However, (and especially with the recent megastorm in mind), there is a situation which has not yet been discussed. And that is, you may find yourself in a situation where you cannot go home (and resupply) for an indefinate period.
The bridge is out,the road is flooded, the riot is between you and home, etc., what ever the reason, you might (possibly) be forced to spend some time with just what's on your person, and in your vehicle.
IF something like that happens, having a backup stash of ammo in the rig might be critical. If you are stranded, for whatever reason, the odds of being able to buy more ammo are pretty low to non-existant, in the locale, and for the duration of the emergency. Its not totally outside the realm of possibility that if you are stranded, you might have to use your weapon, possibly even more than once before the authorities are able to respond. After all, in that kind of situation, if you can't get out, they have a tough time getting in, or even knowing that there is something (else) they need to respond to.
Having a small stash of ammo so that you could replenish your carry gun, and having it with you (vehicle) makes sense to me. Although the odds you are going to need it are tremendously small, if you do need it, you are really going to need it! It takes up very little space, after all.
All else being equal (and it almost never is) bigger bullets tend to work better.