Mikew pretty much hit the nail on the head as did Jerry...but it really depends on what you are shooting...back when I shot lots of concerts, I was pushing Kodak TriX at ISO 1600 rather than it's native ISO of 400...I did it so that I could get the aperture and shutter speed that I felt was right for the shot...I could have changed the other settings but I felt the settings I chose were the best choice...it is all a trade off.
Shooting lots of sports under the lights, something has to give...if I am already shooting the aperture wide open, I have two choices - slow the shutter or bump the ISO...if I get the shutter too slow, I can't stop movement so at some point I am left with increasing ISO as my only option to get the shot I want...Depending on the sport and age group, I may be able to stop action at 1/500 but typically I don't go lower than 1/1000 with most sports...in that case, I bump the ISO and deal with the noise...these days there's quite a bit less noise at a given ISO then there used to be and it keeps improving!
I love my monopod and use it frequently with my 200-400 and even some times with my 70-200 because it does help keep things more stable. The old rule of thumb is to have your shutter speed equal or faster than the longest reach of the lens (so 1/200 or faster for a 70-200) if you are hand holding the image. That is a good starting point to determine if you need some type of support or not, but everyone is different. Some people can shoot at 1/60 with a 300mm and keep it steady...others may find that they need more than the old rule and most folks are really somewhere in between...The only real way to determine what is right for you is to test hand holding the same shot with the same lens at different SS to see when you start seeing camera movement. Lastly, there are times where a monopod or tripod may not be welcome. When I was shooting concerts in clubs, it would have been more difficult to work with a monopod than to work without it...but for sports, I usually try to put my longest lens (whichever it is) on the monopod and carry a shorter lens on my Black Rapid strap...I find it works best for me...