Why change lenses?
I was reading this thread - http://forums.dpreview.com/forums/thread/3284059
It got me wondering why people change lenses in that way? Surely the 100-400 is a fine lens as is the 70-200F4. So what makes people change (in this case to a 70-300L)? I personally would have stuck with the 100-400 and learnt what it is good and bad at then worked around its abilities. Do people just change because they can afford to and just want shiney new toys? Or maybe we change because we thin it will make us a better photographer.
I've had a few lenses over the years and I keep them until they die. I have always tried to by the best glass I can afford at the time then stick with it. It is odd because in my sport of choice (windsurfing) I change boards and sails on a whim but never with photography.
dunno bout the others, I know that I changed a set of 70-200/4 nonIS + 300/4 nonIS to 70-300 L not even half year after the later was shown to the world. and I absolutely love this change - spares a lot of weight and package space.
100-400 to 70-300 - makes sense if the 300mm are enough for 99% of shooting situations and you want/need better IS and size/weight.
I'll also change my 17-40 for its stabilized version (hopefully improved in terms of pure sharpness) the very moment it hits the shelves. love this lens and the way it "images", but extra IS and detail are totally deserving an upgrade for me.
oh... and if Canon ever make a 24-70/4 IS, smaller than 24-105, with better corners in the wide end (24-105 is atrocious at 24mm) and better bokeh - you sure bet I'll be tempted.
in short: to me it's about improvement of those areas where current lens are not stellar. if the improvement is within reasonable price, I'd do it.