The issue I see is to extend the mower with it on the ground you need to be moving ahead. While you can sometimes slide the mower sideways on the roller the relief valve built into the mower will generally open if you try and it won't extend. So you would be leaving an uncut area as you move forward, which may or may not be an issue depending on your situation.
Also, flail mowers don't cut while backing up. In fact Peruzzo says not to back up. They don't say whether that's because they don't cut well or if there is risk of damage to the frame (as leonz said in another thread, the manual is really bad). So if you need to back up to an obstacle you need to have the mower off the ground. If the situation requires it be extended that means lifting it while extended.
Finally, if you are doing really steep upward banks you need to carry the lower end of mower off the ground as you mow. This isn't as much of an issue because in that case the mower isn't actually hanging as far off to the side, so doesn't put as much sideways force on the tractor.