The public seems to demand it. Before a ball has it's release date there are people asking what's next? You can't blame the companies for trying to be profitable because that's why they exist.
Sometimes a new release is a great match for me, and while others aren't bad, they may not be as good a match. I have some old "tried and true" balls that I may never part with.
My advice is to find out which balls seem to give you your best results, and stick with your best layouts on them. Replace a ball when it needs replacing, and not before. The "latest and greatest" isn't as important as you staying on top of your game, in my opinion, because if you're "off" and throwing it like crap, any ball can look like a turd.