I guess my sneak example was a poor choice. The GM allowing players to sneak up on them wasn't to have a predetermined scene take place. The adventurers could have just as easily attacked from a distance, left the room and explored elsewhere, or charged in to disrupt the scene. I'm just talking from a GM standpoint, it seems very few GM's use the "sure you can do this" approach. Yes, the outcome could vary if they roll, but would that really change the scene? Maybe, but doubtful. Because nine out of ten groups will fight the drow in the end. Especially if there is a motive already built in - such as to stop the ritual. That's what I was trying to convey. Yes, it could throw that one party off course, but in my experience that party is rare.
My experience is playing for 30 years, in 6 different states, with 13 different campaigns, and over a hundred con games. So yes, I am using anecdotal evidence, but to me it feels 90% solid.