I think it's pretty case-specific -- depends on the song, the venue, the audience, the performer. Since I'm not a very chatty sort, when I'm performing I tend to keep song intros brief. But some performers definitely make the introductions part of the performance. Arlo is a good example, as is Leo Kottke -- probably a lot of people would have trouble relating to his between-song rambling, but the people who actually go to his concerts tend to enjoy that part of the show, and look forward to it. On the other hand, there's Bob Dylan, who rarely says much of anything -- and that's fine, too.