The answer to the question: "how many D&D books do we really need" depends entirely on the amount of mental space that one is willing to give for this game. Most people do not have the creative capacities to bring a richness to their role playing that these books, and the ones continuing to come out, offer--because most of our day is not spent thinking up new spells and ironing out how attacks of opportunity function. I am grateful that someone, somewhere on a coast is taking the time to do this. That doesn't mean I swallow what they give me wholesale - but it gives me lots of choices I did not have before.
I personally believe the capacity of the humman mind to be creative and imaginative is limitless, and to this end D&D never needs to "run its course." Just like other experiences and stoires from the past, it must be re-translated for each new generation. That is what 3E has done, and this process of refinement will continue. I think that the possibilities for D&D (adventures, settings, spells, campaign worlds, et al) are far greater than some people may realize. Not every product is going to be a zinger, but what strikes one person as fantastic may only seem mediocre to another, or even weird and unusable. Standards for publishing should be high--but also our ability to take other's ideas and run.
