If you're bicycle commuting, go ahead and get a spare bike. Go get the bike and a tire's flat? Grab the other bike, pump the tires up, and go. Something breaks and it'll take a week or two to get the part? Ride the spare bike. Just remember to fix whatever needs it, or you'll soon have two bikes that won't work. Been there, done that. :/
Doesn't work that way when you're touring and 700 miles from home. Been there, done that too.
