Linei, as always, brings up the crux of the issue...
Technically, GMs and writers are not supposed to play adventures that they have run/written.
For GMs, if they're lucky, they are near another GM and can take turns running adventures for each other so that no one has to miss out on ALL the adventures, and just "eats" some of them.
With that being said, I think I will risk saying that I think Shadowrun players are cut from a more mature cloth than some other games out there, and are devoted to the game enough to make that distinction between player/character knowledge enough to play the adventures.
When it comes down to it, if people can't play SR Missions because the GM is afraid he can't play them also, then I'm going to look the other way and let him play the adventure with a different group rather than deny a bunch of players being able to play...
Ideally though, i'd like to see GMs attend conventions or game days or even a second gaming group that they can play the adventure as a player first, and then go to their regular gaming group to GM it. Playing it first will also give the GM a little insight as to how certain scenarios can be run when he's reviewing the text later...
Is that "Kerry" (ie wishy-washy) enough of an answer for you??