Wet, rainy, cold weather activity, or just because.
"Always follow the rules! If you got hit in the head by a band but you don't want to take the 2-minute penalty (for instance), don't sit there fighting it out. Just suck it up and do the time. No one's going to have fun if all you guys end up doing is yelling at each other about rules and regulations. Take your time out, and keep in mind that, in the long run, you're going to have a much better time.
Clean up afterwards. Nobody's wife (or husband) likes to see rubber bands sitting in the chicken soup she had left simmering on the stove while she went out to buy some more carrots. ("Interesting flavor, honey...")
Now you can build your own rubber band machine gun!
"This beauty is a fully functional machine gun with TWELVE rotating barrels and a live action trigger. Loads 12 bands per barrel for a whopping 144 rubber bands that shoot off as fast as you can turn the handle!"