IMO IS is a great addition to pretty much any 100mm+ lens...
All it really does is attempts to reduce the effects of camera-shake during exposure, all an f2.8 lens will do is allow for faster shutter speeds to be used, therefore, less time to be affected my camera-shake...
There are times when you want/need a longer shutter time, to allow for natural subject/background motion blur. In these conditions your options are to use an improved support or us IS. A tripod will offer the best support method, but, there are situations when this will be too restricting, a Monopod will offer some support, but, again you may find this too restrictive and finally IS... IS can allow you to use (with practice) quite slow shutter speeds on quite long lenses and still get nice sharp images.
Some lenses combinations would be pretty much unusable without IS...
This Setup would give an effective 1092mm at f5.6, you would struggle to keep the shutter speed as fast as 1/1000th second even in the brightest of conditions, therefore to yield anything useful the IS has to be used...
This is all based on my experiance and reading, Im more than happy for anyone who knows more about this to come and tell me im wrong...