The trend in the NBA is towards putting the best five players on the court, regardless of position, and sharing the responsibilities on the floor (ball-handling, shooting, rebounding, defense, etc.) with all five players. Why? Because teams will force you to put the ball in the hands of the guy who cannot shoot the basketball. They will force a bad shot. The game is quicker because the rules ask it to be. Transition hoops are best because there's a large area of the floor, around the basket, where you cannot stand. The key. Big men cannot keep up with those players running the floor. They are too big and usually lack the quickness or skill to swipe a ball. They usually commit a foul. 2 of those in the fourth quarter, the big man is done until the 2nd quarter. One more, he's done for the half. They get pulled. Even Dwight Howard.
I think that's fair. Although they still like having a great big man when they can find one (I'm talking hall of fame quality center). The Spurs wouldn't be the Spurs without Tim Duncan. Hell, Duncan's a perfect example of why it's nice to have a good center - rebounding. Duncan was taken out of the game late in game six in order to defend the three point shooters. Unfortunately, it left the Spurs without someone who could rebound. Because of this, the Heat got the rebound and still got the three that the fast people were supposed to defend against.
That being said, I think the days of putting in a mediocre or average center are gone. It's better to go small in those situations. If the Bulls were around today, I could see somebody like Kukoc starting and Luc Longley taking up space on the bench and Rodman under center.