I actually find that I wish I had more than 6' quite often.
I have never had to slow below L3 with the 6' cutter. And that is in the thickest of crap. But I find I have to slow down more frequently due to rough terrain, or obsticals.
I frequently mow in H1 and H2 if terrain permits. Going to a larger cutter will allow me to mow the at the same rate with a slower ground speed (and smoother ride). And there are times where I H2 doesnt even feel like a load, but H3 makes quite a speed jump and I start over-running the cutter and leave a poor finish.
So, since I have yet to find anything that requires using the slowest gears the tractor has, but often times I am at the maximum speed I can go, I wish I had a 7' or 8' twin cutter.