If Mahinmi didn't play last season, when the Spurs desperately needed a big with his physical tools, what makes you think he'd have played next season, with Splitter in tow? And especially against the Lakers? You think Pop would bench Blair AND Bonner in favor of Mahinmi?
I'd have liked to have seen him re-signed as well, but he wasn't going to be the answer against the Lakers, for the simple fact that he wasn't going to play against the Lakers.
In terms of the bigs, I think the Spurs are big enough now. They're still a little undersized, but if Splitter performs to the level most reasonable people expect him to, they should be fine more often than not.
I don't want to say shot blocking is overrated, because it's not, but the threat of it is sometimes more important than the actual block itself. Splitter is and will be a threat to block shots and that changes things. No one is going to think twice about driving against a Blair/Bonner duo, or rush their shot around the rim or make a shot around the rim more difficult than is has to be, but they probably will against Splitter.
As for rebounding, the Spurs were one of the best in the league last season. Bonner is still probably going to play more than he should, but say he's the fifth big more often than not, even though Splitter isn't a great rebounder, just having him take Bonner's spot and Blair playing more minutes will make this an even better rebounding team.
Everyone get's so caught up in what the Spurs don't have, but look at what the Spurs do have. And not just amongst the bigs; look at the back court. This should be the best back court in the league.