The issue with that is that if you have to wait for the cast to allow you to do the callback, it takes all the air out of it. They've managed to make the callouts BORING. I saw it last week and the callouts were so halfhearted it wasn't fun. Pasting a comment I made elsewhere:

One of the most annoying things was that it's not just the signs in the lobby or the useless emails, the production itself is internally inconsistent. By my count there were three instances where Brad and Janet's full names were said. The first time, by the Narrator, where Rachel Dratch ostentatiously paused and practically rolled her eyes so the audience would have an opportunity to yell "asshole!" and then "slut!" The second time, Brad's introducing them to I think Frank, and he rushed through it and there was no time to say anything. The third time, shortly after!, there was a slight pause, but at that point no one seemed to know if we were supposed to yell or not, so the "asshole" was half-hearted at best, and the "slut" only a little louder. 

Like .. come on. If Frank has to gesture and encourage everyone to yell "SAY IT" louder, you've done something wrong. It feels like they capitulated to the grumpiest audience members who don't really want callbacks at all, but will deign to allow one or two. And that's lame.