"but CS2 not only provides image manipulation but also an advanced file and tracking system. "
I'd say that the Organiser in Elements 4 does a much better job than Adobe bridge does , its a lot smoother and quicker and deals with off line files a lot better which is the main reason it sits alongside CS2 on my computer, at least till ACDSee brings out there Pro version photo manager in January
I'd also agree with greypoints veiw about dominance, can you imagine DSLR prices being as cheap as they are now if Nikon or Canon had total dominance but most flock to those two brands simply because of that!