There is another reason for the head and NOT only Ball head should be considered. I use one for the quick release capability. If I need to change lenses (I've had lenses crap out at very inopportune times) instead of unscrewing the thing, mounting a new lens to body and then screwing back in, I just hit the QR and mount a whole second camera and lens with the plate already installed.
I also have numerous occasions (and I shoot a lot where there are things both above and below my feet so that is different as well) where I want to quickly change between fixed focal length or t/s lens and just change the entire camera.
Try shooting on very broken ground such as moto-cross or TT's with a rigid monopod. You'll throw the damn thing away in five minutes. Put a head on, and leave it loose and you will wonder how you ever did without it. The head isn't meant to be tight like on tripod, it is meant to be a guide and keep the camera motion in a range so you can just go right back to the same spot and get, for example, ten riders off a jump with virtually the same background over and over and over in a few seconds.
Try shooting from about ten rows up in a normal basketball arena and you may find the a monopod will greatly lessen any discomfort of swinging the camera back and forth.
There was a mono head I had that was a big chunk of rubber between the camera plate and the pod plate that just let you wiggle the camera around. It didn't last very long, but it works really well and I've never replaced the insert as I haven't had to shoot anything for some time where it would work best.
The adjustable plates whether from Bogen/Manfrotto or ARCA come in very handy when using a T/C, a big 2.8, and wireless controller to handle ganged or preset strobes. Otherwise it can be a bulky unwieldy combo and if a tripod setup isn't practical the movable head really helps.