I would argue that he did. In fact, the Dirk that was playing then is not the same as the Dirk playing now. If not for the improvment Dirk made in his game since Steve left, there is no telling where Dallas would be. To say Nash wouldn't have an impact on very many teams is baseless.
Also since when is Dirk the toughest match up in the league? Dirk is only tough when guys have to lag off him to cover the paint and Dirk is left alone out there. I don't consider Dirk a great post up player either. I can name other players that are much tougher to guard then Dirk. What makes Dirk so tough to guard is that you have to account for him at all time because of his ability to hit the jumper so well for a guy his size.
I was thinking about changing my quote a little, I know Nash made us better but what I meant was he wasn't as effective as he is in Phoenix for numerous reasons. We didn't build players around him and devote the entire system to his strengths. My argument about Nash being as effective on others teams as he is in Phoenix is pretty simple: how many teams are like Phoenix? How many teams have the players that Phoenix has that have been carefully selected to make Nash's job easier? Not many in my opinion. Yeah each team has it's shooters and athletics, but if you have a guy like Nash on a halfcourt team he won't be nearly as effective (ex. replace Amare with Shaq, Shaq would DIE trying to run the whole game playing 40+ min). My argument was also based on the fact that Dirk doesn't rely on his teamates to play his game (and you would argue he doesn't make his teamates better), so he is as valuable on Dallas as he is on any other team primarily for two reasons: his efficiency and his uniqueness.
For the record, Dirk is hardly ever left alone. He gets double teams, he's learned how to swing the ball around to the open man. He is a coaches nightmare, almost no teams have a guy who is big enough and athletic enough to guard all of Dirk's weapons. Marion comes close, but he's still pretty small to cover a seven footer. Like I said guys like Kobe and Arenas are a dime a dozen, guards who can shoot all over. Big surprise. There are players with the skills to stop them, but guarding a seven footer on the perimeter is something the majority of teams aren't ready for.