If you're shooting at a professional level where you need every edge you can get and the mkII might mean not getting the shot while the mkIII might mean getting the shot, then I think it makes sense to upgrade.
But, for the enthusiasts, I'm of the opinion that it probably makes more sense to upgrade every other generation. I have a 5D and while I was tempted when the mkII came out I felt that upgrading to a new model after only three or four years was too soon. I couldn't justify the expense after having the 5D for such a relatively short period of time. I think the enthusiasts, the jump in IQ is really only worth it to make the leap every other generation. So for those who have a mkII, it probably makes sense to hang on to it until the mkIV or whatever, or hold off on the mkIII until we see a price drop that's enough to make you bite.
The other option is that if you have money to burn, then go for it. There are certainly worse things you could spend your money on. Unfortunately, most of us aren't in that category.
