I use the monopod mainly when shooting 300/420/510mm as my 300f4 is not VR and the rough guide of focal length vs shutter speed usually finds me on the wrong side of "typically able to handhold".
If messing around with the big stuff (300 2.8 or similar mahoosive glass) I'd say it was almost certainly required if you are going to get consistent shots and/or no back pain!
My two associates who use the 300 2.8 (and maybe TC's) always use the monopod but belt mounted, which I don't, I stick to sticking it on the floor.