The reason for having game consoles is that you have a single, unified, and dedicated device for playing your games on in the living room for a few years. The natural life cycle that has appeared is 5 years. In this time, developers have time to really find every little tweak that they can apply to each console, squeezing every bit of life out of the machines before they are replaced. If you tried having "something worth buying" every 3 years, most people would barely be entering the market for the old machine when you release the new one and hardware utilization wouldn't be accomplished. Besides, how would the developers take to changing platforms so often and having to support so many a time? I don't think it would end up very well at all. Also, when you are on the 5 year cycle, it gives you extra time to figure out and refine the new ideas you plan on introducing to the next generation.
Now what do I have against the 360 (and PS3)? They don't bring anything interesting to the table.
Why do I want to spend so much money on a new box that really only has improvements on the kind of TV that I do not have and cannot afford?
Online stuff? Whatever. Too much focus is being spent on online junk. I'm not saying that it shouldn't exist, but it should be less of a major focus and more of an added feature. I shold also mention that I'm somewhat antisocial, but that's not the topic here.
Microsoft is set on gaining marketshare by competing against Sony as much as possible (even though I think their machines are too expensive).
Sony is set on making the end-all be-all device in your living room, and is facing the consequences of its bleeding-edge tech (high price, delays).
Both of these companies are seeking to refine the industry, hopefully to their liking.
Nintendo wants to change the industry, the way they always have. They are trying to pull in everyone possible. They aren't paying attention to the current market (because if you do, you end up shrinking it), instead aiming to hook people from all walks of life, by competing with software, interactivity, and low price points.
Obviously, I'm favoring the Big N in this case. If nothing else, because I can equate them as the Apple of the gaming industry, and that's fine by me.
Sorry for the insanely long post.
-Ryan