Yes, Kristin should have been around a lot longer. She could have been a recurring character like Heather Locklear's Sammy Jo in "Dynasty," showing up from time to time causing trouble. Heather Locklear eventually became a regular, and I don't know if Kristin should have stayed for the long term, but she did have a lot of untapped potential.
I remember reading in an interview Mary gave for this website, that she didn't know what else she could have done with Kristin had she stayed, but the cast was like a family to her, so she really missed that when they wrote her off. It sounded like she had mixed feelings about leaving. Being JR's shooter gave her opportunities and she was probably anxious to try other roles, but she was sad to leave the cast.