There are other uses one could imagine, though. (I have one on my JD 1070 and have used it for a number of other applications besides mowing). An air compressor is a nice thing to have on board. You can leave it on full time. If you have a loader, you can move a mid-mount mower up front on the loader frame and power it using the snow blower drive shaft extention. This takes advantage of the easy-on and easy-off features mechanically and adds the nice parts of having a large front mower. A water pump also is a handy thing to have on-board that takes advantage of the high speed pto. Rear units have extra mass because of the need for a gearbox to spin it up. Needless to say, a mid-mount pto generator makes some sense because of the rpm available and the size of it. How about a mid-pressure washer ? All you would need is a fluids tank (saddle tank or a trough in the loader to supply it). Some of this is for people who would need to buy it off a shelf and the rest is for those with a need and some mechanical ability.
To me, the important feature is that it is easy to put on/attach/hook up, can be left on without have it in the way, keeps the rear pto, hitch, and drawbar free for other service and makes the neighbors say "Hey, I wish I had me one of them deals". My air compressor is a radial type (so it has some ground clearance issues, but I see there are now some opposed piston types that ought to work and stay hidden. No need to run it at a speed other than idle, too. Better than carrying an air tank and can drive a sandblaster (not to mention airing up a tire out in the middle of NoWhere).