TJSpiel says:
I would change the tube and be on my way a few minutes later. In fact I think I've actually done that. If I remember right I ran over some glass the recyclers broke in our alley. I chose to change the tube rather than hop on the winter bike because that bike would have probably needed air in both tires anyway and in general I just don't enjoy riding it as much.
What happens if you awaken late, get half way to work and have a flat? What if you have some other mechanical issue on the way to work?
Bikes are fairly simple machines. If well maintained, they're going to be pretty reliable. Having a 2nd bike as a backup can be handy but it would also require some work to make sure it's always ready to go. Personally I wouldn't keep a 2nd bike just as a backup.
TJ, you've just got to admit the fact that you've got skills. Skills most commuter cyclists can only dream about!
Unfortunately TJ, most cyclists just aren't that mechanically inclined. That goes for commuters too!
ThermionicScott says:
+1. I'd even say that if you can't fix a flat, you have no business venturing out on your bike.
Personally, I believe that there are many cyclists that can't fix a flat. IMO, for quite a few commuter cyclists and roadies, that's the total extent of their bicycle mechanical expertise.
However, even if they do know how to fix a flat, that doesn't help their job situation, if they awaken late and have to take an extra 15-20 minutes trying to fix a flat.
Bottomline...It can do nothing but help to have a spare bike, if you're just the average commuter cyclist.