The frame change to glass in rubber could have been a cost-saving exercise, though it could have been due to leaks
It must be quicker and cheaper to fit rubber than the Moydon frame, and no screws etc. though as you say, the roof then needed strengthening ribs above the window, I assume to stop the glass popping out of the rubber when flexed!
Interestingly, one of the windows in mine is perspex, the other glass - either they were using up stock or it was replaced later due to breakage.
I must admit, after replacing the rubber seal on mine, I would have preferred the frame solution - the skin of the roof is so flexible it is really quite difficult to push the glass into the rubber, and is certainly not a one-man job
Cheers,
Robin.