They need to go. Anybody who says just move to get a view must not have gone to Elton on Sunday, as once you're in a crowd like that there is simply nowhere to go.
We were quite close to the front, maybe 1/5th of the field back, with a really good view of the left hand screen. All was well and then just before he started someone put up their 3 flag pole and for my wife who is a 5.5ft it was all over. She saw nothing for the whole set as the screen was totally obscured. Me being taller I could just about see bits, I could have swapped places with her but the girl behind my wife was the same height as her and I didn't want to block her view even more.
All you need to enjoy a set at Glasto is your eyes and ears, anything else like flags and flares can do one.