The first reason is movement. In my experience, underglazes and colored slips (both contain clay) tend to adhere better to surfaces. I've had no experiences with it running under a final coat of clear glaze. This gives me sharper lines in my carving.
Secondly, I want strong, intense colors in my upcoming prototypes and projects. Something like this by Sharon Bloom:

The following is a wonderful majolica piece by Judy B. Freeman. As opposed to underglazes, majolica is glaze -- much more closely related to glass than underglaze. As such, it has a softer, more watercolory feel to the coloration, which definitely works on this piece, but which I don't think I could pull off with the designs I have from David:

Of course, as this life tends to work, I'm sure there will come a time when I decide I've had enough of underglazes and get brave enough to start working in the world of majolica.
Yesterday I threw a couple of plates to work on. I'm hoping to get them trimmed out today. Then, when my anticipated delivery of underglazes comes tomorrow, I can get to work on them next week.