This question is almost as arbitrary as the "does Summer League matter?" question. Where does it come from? Well, simply put, my personal experience is from overly zealous fantasy basketball team managers who are trying to get the jump at the next sleeper in the fantasy draft. Scott Sereday at 48minutesofhell did an analysis showing that teams that typically did well in the preseason do well in the regular season too. While I can't argue with the numbers, my gut intuition leads me to believe that they don't necessarily correlate very well. I'm not actually going to go into the Summer League example as that's a whole other beast that I don't want to deal with right now, but the question really does boil down to, what difference do these games make if they don't count for anything? Not having actually seen a whole preseason game, I can't really say anything definitively, so while I don't necessarily believe that preseason wins will correlate into regular season wins (just as regular season wins don't correlate necessarily to postseason wins) I will say that preseason games do serve a very important purpose for each team. So does this mean that San Antonio's one loss to Houston isn't a big deal? That our one win over Miami isn't a big deal? Well, I'll go with the cop out answer: yes and no.
Realistically, Duncan is going to play more than half of the game, and LeBron and Bosh are both going to play for more than a single quarter. So what does the preseason do? Well, in the end, I don't believe that we can take too much from the simple fact that the Spurs are 1-1 right now, but rather I think what is to be taken from it is what happened during the 96 minutes of play. There are two important things that I think can be gleaned from these preseason outings: first, efficiency, I don't care who scores 20 points in a preseason game, what I do care about is how he did it. Secondly, on court chemistry, how well specific pairings or even entire lineups play with each other. These two give coaches better ideas of how to make rotations work and help fine tune who to keep and who to cut.
The first part is efficiency. You keep someone in the game for long enough and he gets enough touches, he'll put up 20+ points. Just look at Allen Iverson. Dude averaged 26.7 points on 21.8 shots per game. The man was averaging 41.1 minutes per game. That means that he's sitting for only 7 minutes. He can't come off the bench though. What coaches look for in giving younger rotation guys more burn is who can be effective in limited minutes. You know that Pop isn't going to give James Anderson more minutes over Manu or even Richard Jefferson, but what Pop is looking for is whether or not Anderson can be effective in the minutes he's given.
The second part is meshing. We knew that Zach Randolph could score in the post, we knew that Eddy Curry could score in the post, when Randolph got traded to the Knicks then, there must be double the scoring in the post, right? Wrong, Randolph and Curry ended up cannibalizing each others' games. What Pop is looking for here is how well pairings work, can DeJuan Blair and Tim Duncan be effective on the floor together, what about Tony and Hill, who does RJ play the best next to? It's these kinds of questions that in-team scrimmages can't answer but semi-casual preseason games can. That's the beauty and the benefit of preseason.