To me, VR/IS is only important when shooting with handholding with a relatively slow shutter speed. For example, when you shoot forest birds or concerts or in house events, it is not uncommon to use a shutter speed around 1/200s or below. If it is more convenient for your to handhold your lens and camera in this situation, a VR/IS definitely help.
Another common use of VR/IS is panning with handheld. When shooting car races it will be more convenient and flexible for you to handhold the lens then pan and shoot with a slow shutter speed.
VR/IS is however not necessary if the shutter speed is faster than the inverse of your lens' focal length, roughly speaking. Take you 300mm f2.8 as an example, if you can maintain the shutter speed faster than 1/300s, theoretically you don't need the VR/IS at all.
Moreover, when used on top of a sturdy tripod, you actually need to turn off any VR/IS in order to get a sharper image when slow shutter speed is used, as VR/IS may cause minor vibrations on a tripod.
In shooting birds in flights, VR/IS is not important at all as usually you need to maintain a shutter speed as high as 1/2000s which far beyond the inverse of you 300mm lens' focal length.