That is the whole point of a demo being the best it can be. Producers want to hear how a song WILL sound. Gone are the days when they heard beyond the rough demo to what a song could sound like. Most demos pitched are as good and radio ready as the finished article. Sending out anything less minimizes any chance of a cut.
It does not matter whether it is a capella or a full orchestra production the rules are the same. The song must be produced to the best standard it can be in every way. IMO it is probably harder to send a simple a capella version than a full production as when it comes to bare vocals there is nothing to hide behind. The performance, recording and production must be top notch.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055