Nuru,
Although I do not have one myself, I have seen some work done by the 72" mid mount mower, and it cuts really nicely. I assume that you are mentioning the non-7 iron model. The 7 iron model is a little nicer and more heavy-duty, but it is not that much nicer than the older standard 72". These are heavy mowers, and although they cut as well as a lawn tractor in most cases, they can scalp some if the terrain is really irregular or one is cutting too short. I found it interesting back when I had a 72" rear mounted finish mower on my machine. My parents have a large, highly-finished lawn and my father was concerned at times when I mowed for him that I might do a bad job, or the tractor I have (3720--same size as 4310) would scalp, etc. due to the machine weight and tires. Surprisingly to him these big machines tread as lightly as a lawn tractor and actually cut BETTER than most. He had at that time a little 335 JD GT and the 3720 would easily do a better job on his lawn when I mowed it. I think your concerns are valid, but I would say the work these machines do with these heavy duty decks is quite impressive.
John M