To prevent clients from being billed twice for essentially the same service.
or
Twenty. One to hold the bulb, two to turn the ladder, and 17 to be on the guest list.
or
the barman says "Oi - get out! We don't want your type in here"
or
In their Sleevies!
or
Three:
One to write the light bulb removal program,
one to write the light bulb insertion program, and
one to act as a light bulb administrator to make sure nobody else tries to change the light bulb at the same time.
or