Why do many instructors avoid giving personal introductions? I recall that as an undergraduate, very few of the older instructors ever presented basic details about themselves. While I can understand how most would not reveal much about their family life, I cannot understand why few told about their academic background, research interests, work history, or other details which seem connected to the course. This was not the case with the younger instructors and graduate student teachers. Now that I teach undergraduates, I wonder if these older instructors knew through experience that it is best to keep quiet about one's background. Is there a good reason for instructors to keep back such information?
I am not sure whether this is in any way related to the age of the instructor, but the primary reason that most people I know don't do a "this is me and my vast experience and research results" style introduction is that it is all too easy to come across as bragging. Even if you have the best intentions and just want to explain your background to the students, so that they can judge your qualifications for themselves, it may sound to them like you are really just showing off.

Most students assume that the instructor knows what he's talking about, until they have solid evidence to the contrary. There is really no reason to explicitly talk about your experience beforehand.

That being said, if you feel like a short introduction of yourself will add to the course, go ahead. In the grand scheme of things, I am sure whether you talk for 5 minutes about yourself in the beginning of a course is the tiniest of issues. What I would avoid is what one industrial lecturer in my alma mater did. This guy spent 5 - 10 minutes of the beginning of every lecture talking about himself and his company (and was ridiculed a fair bit for this by the students :) ).