While attending a Blue Jays game in 2014, I noticed that the sports photographers were using monopods to hold their cameras and huge telephoto lenses. So, I bought a monopod and have been using it with my Sigma 150-600 mm lens. But I almost never see other photo enthusiasts using this accessory; they either shoot handheld or with the camera on a tripod. Why is that?
Like you Maria, I also own a monopod, a Manfrotto pro model, and I do use it often when shooting in daylight with a tele lens that’s not excessively large/heavy (such as the AF-S NIKKOR 200-400mm f/4G ED VR II model). But I agree that the majority of photo enthusiasts and professionals prefer a three-legged accessory. When I photograph birds in Florida or closer to home, virtually all of the large telephoto lenses are mounted on a Manfrotto or Gitzo tripod.
Of course, three sturdy legs are definitely better than one when you will be shooting at a long shutter speed where blurring from camera shake would be problematic. In a night scene, for example, a tripod is ideal since it allows for shooting at a long shutter speed, using a lower ISO level for optimal image quality. (It’s also ideal when you want to make precise compositional adjustments in landscape photography, for example.) When the camera is fully supported, you’ll get razor sharp images of buildings even at a 10-second exposure. Because of this aspect, I always use a tripod when shooting in low light, unless that accessory is prohibited at a certain location, such as a cathedral, or impractical to use, as on a crowded street.
However, at a high-school football practice or for go-kart race photography in daylight, a tripod has no real benefit in terms of camera stability. Since we typically use a shutter speed of 1/1000 sec. or faster (to freeze the motion of the subjects), we can get sharp photos even when hand-holding the lens. Of course, if your camera and lens are heavy, a monopod is useful; there’s no need to hand-hold the equipment for hours. And it does allow for convenient panning (moving the camera/lens to make a series of photos as a subject rapidly moves past the shooting position).
Granted, even when shooting at very fast shutter speeds, most other serious photographers will be using a tripod with a long lens unless it’s prohibited or impractical to use because of limited space. Why? Because it’s their habit, based on always using the most advanced technique. You certainly cannot argue with that rationale, but since you are satisfied when using a monopod with your 150-600 mm lens at fast shutter speeds, I see no compelling reason to change your technique simply because “everyone else is doing so.”