Most colleges in india ask for full-time faculty members.
The reasons quoted are...more commitment, better focus, more dedication, more time...etc., etc.
I am sure many practitioners would love to share their knowledge and their clinical expertise with undergraduate teachers.
In fact, i can vouch for the fact that most part-time teachers/faculty members make better teachers.
They take time off from their practice time to share knowledge..and they are more focused and better equipped to teach, from the point of view of treating and dealing with patients in the outside world.