At 6'11 245, Splitter has exceptional size for a PF, which allows him to match up with guys like LaMarcus Aldridge and make them work for points. In contrast to the first-round series against the Rockets, where he could shoot over the top of Terrence Jones like he was a chair and go around like Omer Asik like he was playing in molasses, Aldridge has struggled against Splitter, whose just long, fast and strong enough to play straight up D on him.
Splitter's not a shut down defender by any means, but he's an excellent weapon for the Spurs in the West, where they have to go up against top-tier PF's in almost every series. He did a great job on Dirk Nowitzki in round 1 and he was absolutely vital for them in last year's Western Conference Finals, where he matched up with Zach Randolph. With Splitter as the primary defender on him, Aldridge went 12-25 and 6-23 in Games 1 and 2.
Aldridge is going to have better games as the series goes on, because he's one of the best scorers in the league and not even Splitter can really contest his turn-around J. What Splitter can do is hold him to 23-27 points on 45% shooting while allowing the rest of the San Antonio defenders to stay at home on the Portland shooters. That's a lot of points but it's not the 28-30 points on 55% shooting he was getting against Houston.
Just as important, Splitter is skilled enough to not destroy the Spurs spacing on the offensive end of the floor. Houston was very reluctant to go with Dwight Howard-Omer Asik in the frontcourt because neither guy can step out and hit jumpers or make any plays from the high post. Splitter isn't an All-Star or a primary option, but he's a skilled big man with a good feel for the game who can play high/low with Tim Duncan.
He's not a guy who can command a double team in the low post and he's not really a guy who can punish teams for going small against them, but almost all the top teams in the West have high-level PF's who force you to play two-post for most of the game. Without Splitter, San Antonio doesn't have the personnel to match up them. Memphis doesn't beat them in 2011 if Splitter is playing big minutes.
So while Splitter wouldn't be that useful in a rematch with Miami in the Finals, San Antonio is going to need him if they are going to make it that far. He's not great at any one facet of the game, but when you are at 7'0, you don't really need to be. Splitter is decent at offense and decent at defense - he doesn't have any huge holes in his game, which makes him more than worth what the Spurs are paying him.