I recently had a correspondence with a prominent artist manager who represents both Label and Independent artists asking this very question, which is one that I get from time to time, Here are some thoughts on this issue as well as where I fall out on this question having been both an artist/content creator, label owner and mastering engineer.
To Artist Manager-
I am getting this question a lot. More and more people are asking for this to be done... it is primarily because of synch and licensing. Many Licensing companies require having instrumental mixes readily available in order to secure synch licenses for film, tv and advertising. The thought goes like this. If a song gets synched and the vendor wants to use an edit between the final master and the instrumental of the song than in order for them to be able to make a seamless edit the sonic character of these two files will have to match as closely as possible. Thus the thought behind "mastering" the instrumental tracks. This is one way to go... the other way to go is to send the vendor "un-mastered" mix files of the song and to have them edit between the files that were supplied by the mix engineer. In many (if not all cases) this will be more than sufficient. My thinking goes, whomever does the post-production audio for film and or tv will likely set his or her own levels and may even apply some processing to the audio in order to achieve the desired results for the synch. As well the end format and resolution is higher for some video formats than for CD audio, so giving the Video post-production house tracks that are mastered for CD or (HORRORS) an mp3 could actually degrade the potential sonic presentation of your track. I should state however that not everyone shares my view on this and some would claim that the difference between the mixed file and mastered audio file is so significant that they would not feel that to deliver un "mastered" files would represent the full integrity of the finished work. So there you go. In my opinion if your record is being mixed by A level talent and they are supplying TV and INST mixes as High-Res Digital files I would use those for Film and TV synch and licensing and save your self a little dough and not bother mastering the instrumentals... If you are self-producing and self-mixing your project and find that your Mastering Engineer is doing deep rescue work to your mixes you may prefer to have him Master the Instrumentals to ensure continuity among potential formats. Mastering dollars in my opinion are best spent on preparing final mixed records for duplication and distribution for CD, LP and the internet.