The idea behind the whole game is groups of (40/25/10) people getting together at a PRE-ARRANGED time and place in the game to do something. The something would generally be highly organised in advance, requiring precision, timing, knowing your particular role, strategy and TIME.
Often if people want to do this pre-arranged thing (which is fun undoubtebly), they must "sign-up" in advance. If they sign up, and they get a call an hour before from mates... well, they have made a commitment to the other people (and have taken a spot from someone else who would love to go). Its all about that. If they dont sign up, they cant come.
You wouldnt make a commitment to your mates and then turn around an hour before and say "sorry heading out bye!".
Doesnt mean I agree entirely, but im just trying to let people understand.