Always get the GAC. If you do have a need, it'll be handy. In our case, I usually wear a lanyard with our passes and the GAC in a pouch. Quite a few CMs notice the red and white corners BEHIND our passes and ask what our need is before it's out of the pouch.
The
MVMCP I did last year was right before the crowds (either the 17th or 19th) descended on WDW, so we didn't even need it that night. We've done
MNSSHP three times now, and even on Halloween '04, we didn't need it. It was pouring at an early one last year and I think all the attendees were in Tomorrowland

, hence the crowds at Buzz.
We couldn't go to the parks during those peak weeks, so I don't know exactly how crowded they get. However, I've been during the summer many times and found the lines to be moving and manageable with my son's needs. We're able to get by if there's not a lot of standing in one place with crowds all around him. If the line is moving constantly or there's space to move around, he's okay.
Tacomaranch, ain't it the truth about Autism? My son has Asperger's and it's totally different from Kanner's. We end up explaining a lot. My nickel speech is that he looks like a 10 year old, but emotionally, he's 3. He's fine with everything in the parks but Star Tours and Mission Space. He's the biggest Star Wars nut you'd ever meet, but the motion of the ride freaks him out.
Suzanne