I think part of the reason we see so many no-shows is because of the open registration.
For example...
Let's say a certain squad wants to fly together in an upcoming scenario. They plan for weeks before the sign up that they will all try to get in the same unit and fly together.
When the registration finally opens, only a couple of them are able to sign up right away, and the others can't make it until the next morning. During that time many other players have signed up, and the scenario squadron is now full, so the other guys (who wanted to sign up with the first two) now get stuck as reserve, or picked to fly in another unit, or even worse, assigned to the other country entirely. Maybe even a couple of them decide not to sign up at all, knowing they have virtually NO CHANCE of getting the assignment they wanted.
Furthermore, the other pilots who signed up did so just because they could, even though many of them only did it to reserve themselves so they could fly if they FELT LIKE IT that day. And let's not forget the 2-week wonders, who sign up for AH just so they can register for the scenario.
In the interum, the group of guys who originally wanted to fly together start having second thoughts. They ask the CMs for reassignment and are told no, sorry, you can't.
Finally, come game day, a couple of them just say screw it. Why fly in this scenario when I can't do what I wanted to do? I've wanted to fly this certain slot the whole time, and flying this other slot doesn't interest me.
Then you have the no-shows. The guys who don't care anyway. Even through repeated Email from their flight leader reminding them of the day and time and mission briefing. They only signed up because it was easy and they loose nothing for not showing up.
So now you have a flight leader that is HOPING his reserves show up and that they have been keeping up with the mission briefings and other pre-game Emails. What probably ends up happening is the flight leader has to spend time trying to recruit from the walk-on pool and then trying to breif his newbies. The whole mission becomes a big CF because you have a FL trying to organize a flight telling all his pilots what channels to tune, how to join up in the mission, how to get convergence set for the plane type, how to manage fuel in flight, follow the formation, etc.
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By making the first day or two of registration open to players who are willing to PAY MONEY for the privledge of choosing their country, squad, aircraft, etc. you are increasing the chances that the scenario will get a better turnout and run more smoothly. I believe this because the guys that signed up for money have invested something into it because they sincerely had an interest and wanted to participate.
Also, by giving the CMs credits on their AH accounts, it is like paying them for setting up the events and getting them running.