Keven520, I can see where your coming from but I disagree. I think that with the popularity of the American Idiot cast album both with theatre fans and non theatre fans alike, the full cast version of 21 Guns sans Billy Joe on vocals is getting its chance to be heard.
Also, wasn't there a version of the song recorded without Billy Joe on vocals? I don't mean the one that is on the cast recording. I thought that there was a version of the song recorded with Matt Caplan prior to his being replaced. However, I could be wrong.
Regardless, I always viewed the 21 Guns single as more of a promo for the show and a way to showcase the new orchestrations etc.
There had been times during the course of Movin' Out for example, where Billy Joel would visit the show and get onstage and do New York State Of Mind. Now, by having Billy get on the stage of the Richard Rodgers and sing a song (granted, one that wasn't in the show) have the effect of him not letting it go and stepping back? No, it is viewed by most as a fun encore and something that the original artist did for fun.
"If you try to shag my husband while I am still alive, I will shove the art of motorcycle maintenance up your rancid little Cu**. That's a good dear"
Tom Stoppard's Rock N Roll