I was given a prehung door unit. Two 24" framed glass doors meeting in the center with a 24" fixed glass door panel on each side. A total of 8 foot wide for use in my backyard office.
The doors are excellent. But looking at the pre-hung part, the bottom plate and 1/2 - 1 inch of the door frame is rotted. This is a quality set (except for the bottoms). Could I replace the bottom few inches of the 4 jams? Could I cut an inch off the jams and doors and create a new bottom of the frame? Is it worth trying to repair?

You have stated that the doors are excellent. I would try to repair them. I would say that cutting out a piece of the jams and doors would be better left to someone who knows woodworking and touch-up/refinishing, unless you are able to do that kind of work yourself.

Just a small piece of repair and touch-up is usually worth it, if the piece is otherwise in excellent condition and serves your purposes.

A pre hung door uses the same components as a stick built one. Buy a clear 1 x 6 and matching moldings and build the door frame yourself. Shim the rough opening as needed to make things fit. Finish to match the doors.

Pre hung doors are a huge labor saver, but nothing a good worker can't duplicate.

I definitely agree with Labman on this one. I would remover the doors from the damaged frame, install a new frame in the rough opening and re-hang the doors. You can buy a "door frame kit" that has all of the components pre-cut and ready to install.