I just can't buy this in a night-in-night-out basis. Crowds just generally aren't that eclectic that they want a different feel/genra/tempo for the every next song. See what I wrote above. Pros can walk into a room, see the crowd, and pretty much guess what is going to work and what isn't, and be right most of the time. That's not to mention the fact that I bet for most of us in established bands, half or more of the venues we play every year are ones we have played in the past and know what the crowd there like. In those cases, it's just silly to dis a pre-arranged script, given what can go wrong if you don't have one (long awkward pauses between songs, guys not hearing the call of the next song and starting the wrong song in the wrong key, not having the right effect dialed in, etc. etc. etc.).
Again, that doesn't mean that a true pro won't know when the expected script is failing, and deviate until the right songs are found.