I am a self-admitted Maverick fan, but I believe that the Mavs are not only the best team in the NBA, but they are also one of the more impressive teams I've ever watched (frame of reference is Michael Jordan's Bulls, Magic's Lakers and Bird's Celtics). Most of the games against San Antonio are always incredibly well played games and whether you are a Mavs fan or not, it's the best professional basketball has to offer.
Wes Cox who writes the very well done Mavs Moneyball had a post which linked Pounding the Rock, a Spurs blog that notes the declining play of the Spurs because of their aging role players. I think that it's understandable that this is an issue for Spurs fans, but in reality, that's pretty easy to fix. Most NBA teams don't spend a lot of money on the role players and it's usually pretty easy to get out from under those contracts.
The Mavs are simply just a better team right now, role players or not. Their players are just better than the Spurs' players.