Ive been following this thread with interest and have thought about responding, but havent because I wasnt exactly sure how I felt about the topic, I see both (or all 5) sides, somewhat. Then, today, I figured it out:
Substitution requests should be just that: requests. Its nice if the restaurant can accomodate the request, and the simpler or deadlier (nut allergies) the request, the more likely it should be to be done. However, the asker should be asking with the spirit of a request, which means NEVER getting upset or offended if the restaurant says no. Yes, it would be nice if an explanation is given, but it shouldnt be required. If the answer is no, move on to the backup item you had planned to order in case the request was denied (you did think that far ahead, didnt you? I think it would be rude to not).
Spirit of a request. That is my position. If you are making a demand instead of a request, you are out of line.