I'm not too sure why anyone even requests what side they would like to fly on or what type of aircraft. That particular request sheet does not seem to be used. 
If you mean not used by the COs, well it's not required but they need not complain about getting a raw deal.
If you mean by the CMs and CiCs:
I'll let a CM answer for them, but I'll take a swing at the CiC part of it.
There are several factors at play here. First you have the ones that requested the other side and did not update for the rides available to the side they got. Then there's the ones that don't request any ride. And sometimes you have the ones that let it ride so their requests are for rides not available on either side.
Next is the selection of what is available vs what needs to get done and the division of manpower to fill the seats. A CiC should also look at what each squad did in the last 3 frames (in my case 6 frames) to make sure everyone get a fair turn between egg chucking and fighters. This in itself can reduce the number of squads that get what they asked for.
Next is a CiC personal way of doing things. I get the impression that a large squad split between bombers and close escort does slightly better than 2 or 3 squads combined in a package. I also choose not to overwhelm CAP but only to distract and draw them down. And I've see 3 smaller squads split defence into 3 areas: sweep tanglers, escort strippers and bomber killers. So I tend to think along those lines. All this can further erode the ride request list.
Then it's back to the rides requested to see if I can swap rides or duties to try and get them what they asked for. In the end only half ask for anything that I could really give them and of that only half get what they ask for.
But would you rather not be asked what you would like to be put in? If nothing else it gives some the chance to volunteer for bombers or planes that are fun for a particular squad.
And not getting what you what can have unforeseen benefits. I learned a few things about the 39Q this last go round.
