I may have misunderstood Leeneia's original post, but I thought she was talking about actual rehearsing and practicing, not the usual noodling and tuning that one expects before a orchestral concert or musical play or some such.
I think we are all accustomed to the seating of the orchestra and the loosening up that is part and parcel of the experience. But what if they are, as she said, practicing all the tricky riffs and various parts of the musical score they are going to be presenting? Hearing a scale used to warm up doesn't bother me, but hearing the trumpet go through his/her fanfare would sort of bum me out. As with Leeneia's food analogy, I would expect the knives to be sharpened, the food to be prepared, etc. But when it came to the final presentation, I would not want the chef to sharpen his/her knife at the dinner table, nor should he/she disembowel the salmon over my plate, no matter how skilled the chef may be. Warming up is okay, but I think the final "nerve settling" should be done backstage, or at least before the audience starts to be seated.