I have a dedicated lawn tractor (Ventrac 3000) and a CUT with a MMM (NH TC24D) and find that I use both. But I would prefer to just have the CUT.
The Ventrac mows fast, gets in tighter places, goes on the slopes that the TC24 can't manage, etc. The TC24 has the MMM deck pinned in the UP position much of the time, but I often find that I will be out doing something on part of the property, and while I am there I just drop the deck and mow it. If I need to play with the FEL, etc then I raise the deck and do the work.
My property has several dozen small trees that a CUT or even a sub-CUT can NOT get under so the Ventrac always does those areas. I also have a couple killer slopes that require the Ventrac's stability for mowing.
However, if I had a property that was somewhat differnt, I would have saved the extra money and just used the TC24 for mowing. The cost of a MMM is equal to the price of a cheap lawn tractor (about $2000) and for my money, the MMM does a better job and doesn't require the added maintainence, added storage space, etc that a separate lawn tractor requires.
I would also agree with a previous post that a low priced lawn tractor will not mow as well as a high quality deck on a CUT. When I went shopping for the Ventrac, I started out looking at every brand of lawn/garden tractor on the market, and quality of cut was important. I ended up spending $9850 for the Ventrac, the deck, and a front slip bucket. The quality of the cut is amazing, it never scalps even on bumpy terrain, it lays down perfect lawn stripes, etc. I've tried JD 2xx and 3xx tractors as well as Cub Cadets (which I also own) and the Ventrac is so far superior to the other lawn tractors that it justifies the added price. I find the MMM on the TC24 is not quite up to the quality of the Ventrac, but is better than most of the lawn tractors by a pretty long shot.
I would love to get rid of the Ventrac (and I really enjoy using it when Mrs_Bob lets me!) but the reality is I need a tractor and a mower unit. If my land was different, I would only have a CUT and probably be happier.