After all is said and done, I agree with your criteria. I do believe the MVP should be determined by balancing team success with individual responsibility for that success. But even if that is the case, I agree with Darrin and O_V: Nash's' team was not the most successful in the NBA, and Nash was not the player most instrumental to their particular team's success.
There were three teams this year more successful than the Suns (Pistons, Spurs, Mavs).
There were several players more important to their team's success than Nash (Kobe and LeBron unquestionably, Dirk arguably).
In order to choose Nash over Dirk (the only person who fulfills both criteria of top-three team success and top-three individual responsibility), you have to weight individual responsibility way more than team success. I guess I just disagree with how you've weighted the two factors. I think they should be taken on more equal footing. Take the top teams in the league, then take the top important players, and see where the two overlap. Last year, they overlapped in Phoenix. But this year, they did not.
I think Nash wins this award because a) he won it last year and b) everyone predicted the Suns would suck without Amare. In order for the voters to lend credibility to their votes from last year and explain away their horrific errors in predicting the Suns' performance they have voted for him again. It's kind of an ego thing...they don't want to admit that they were wrong, so they find the one excuse that will make it okay, deluding themselves into thinking "Nash had a better year this year than last, and that's the reason why Phoenix stayed afloat in the Western Conference." But as others have pointed out, Nash didn't have a better year, and his team didn't do as well, and there was much stiffer compe ion. The ego factor is the only thing that explains this award.
On a side note, I appreciate that you haven't been sucked into a flame war with any of us SirChaz...a few of us (myself included) have said some pretty derogatory things about you and your boy Steve Nash.