I used to think this too but then I once saw one of the mechanics from my FBS replace a driveside spoke on the road in about five minutes with the wheel still on the bike. He was leading our group on one of our Friday night rides and one of the riders broke a rear spoke. He had the old spoke out in about a minute, strung the new one in place the next and got it tensioned and everything pretty close to true in a couple of minutes. The rider said her brakes pulsated slightly when she lightly applied them but other than that, everything felt fine. I know it was only five minutes because I was on a schedule that day and was watching the clock. It wasn't a permanent fix but it kept the group rolling.