A couple of weeks ago I was running a bit late for work. Grabbed my bum bike and ran out the door. Down the stairs and into the alley. Swing leg over and push off. Handling seemed a bit off as I slid sideways down the alley so I looked down, flat rear tire. Run back inside, grab other bike, transfer trunk bag and I'm off to work. Find out later tire and tube had big cuts from glass. Fixed it that night. Always have a spare/beater bike if you can! Thrift store 10 spds are the best!

EDIT: Yes if it had been a mile from home I would have fixed the flat and continued on my merry way. But for those times when you break a spoke on the way home from work, get roped into dinner with the in-laws and don't have a chance to repair the wheel, spare bike saves the day.