I think this answer is really conference specific. At my field's annual conference, everything is closely timed. Each paper in a session is given a specific slot of time so that someone can come to hear a certain paper. Any introductions at the beginning just take away time from the first person. Usually the chair gives a quick welcome and then reads the title and institution of the author. As chair, I think the more important job is keeping everyone on time.
Same in my field. In fact, if the chair did anything
except give title and author, and then cut off discussion at the time limit, it would be seen as pretty impolite, and even incompetent.
This means, of course, that chairing a session you're in is more or less trivial in my field, as long as you obsessively keep yourself from going over time (or get a friend to signal you when you're to be finished). It's still not normally done, but it wouldn't be anything to really stress about.
On preview: Introductions like the ones everybody keeps talking about are generally given before plenaries in my field, but even there my field's intros are a lot briefer than the ones I've heard when I've gone to other fields' conferences.