They gradually work through different phases of a dancefloor, having the place ready to rock by the time the headliner comes on, but not actually blowing the place up.
Philosophical discussion here...
I agree with that statement for clubs. One-room clubs, specifically.
However, I've noticed that for multi-room clubs and festival situations... that doesn't fly so much. People will wander in when they hear something they like, and instantly wander out the second they lose interest.
...Which is why trance is so devastating to an untrained dancefloor. It's all fun and games until the
Epic Breakdown...Crowd: <

>
30 seconds later... it's just starting to come back up, but...
Crowd:

Now, I'm not saying that openers need to crank out the big anthems for their sets -- or that trance nights at festivals are bad.

I'm saying that asking for Jerry Bonham and Evil 9 sets may backfire when the night comes.
Shoot, I distinctly remember seeing
Tiesto clear the floor his first breakdown. People were getting completely into it, progressive and all, and then the breakdown hit, and "Oh, yeah, I can hear Busta Rhymes playing on the other stage."
Exodus.
Is there something different about the Cornerstone night that makes it different? The most successful opening DJ set in that environment I've ever seen was Dieselboy (at the same event Tiesto mucked up). He grabbed 90% of the people at the festival just by keeping the subwoofers constantly quiverring and by having an extremely good MC to literally explain what was happening when the rumblies ceased.
Especially at a rock concert, shouldn't the idea be, within the constraints of not stepping on the headliner's toes, to
inventively hold peoples' attention as much as possible?