<p>I sing both professionally and with a couple of select amateur groups, and I have also directed. Vocal sight reading was one of the best skills I picked up in college. While most a capella groups eventually memorize their music for performances, many use printed scores for rehearsals. The ability to spend time on polishing and fine tuning the presentation rather than on learning the notes is priceless to a director. </p>
<p>You can work around the lack of this skill if your singers are willing to put in the required time outside of rehearsals, but sight reading allows the group to try a lot of new repertoire early in the rehearsal process. It becomes much easier to weed out the pieces that are not going to work for the group without having to invest a lot of rehearsal time on stuff that you will never perform.</p>