Well how about when the sickle is in the elevated mow position where that "7' Moment of Inertia" is dangling off to the side. Keeping the left front tire on the ground would be the concern. I agree that if in the vertical transport or in the mow position with the bar on the ground ready to cut/cutting, no big deal.
On hillside mowing, putting liquid in the rear tires, about half full, helps to lower the center of gravity as does spreading your tires out as far as you can. Other thing would be to run the hills with the weight above you. I realize that if the mower is on the ground no biggie, but what if you had to raise it to get over a big rock or something, you would want that counter balance above you.
I ran 3 of them over the years and now run a drum and the dealer of the drum, which rides just like a disc or sickle, cautions you to account for what I said in selecting the size mower.
HTH,
Mark