The current Spurs frontcourt consists of 7 players, Tim Duncan, Antonio McDyess, DeJuan Blair, Ian Mahinmi, Matt Bonner, Marcus Haislip, and now Theo Ratliff. Out of those five, neither Blair nor Mahinmi has played significant pro-ball, additionally Haislip hasn't seen daylight in the NBA since 2004 where he was subsequently dubbed a bust. So what we have, excluding Ratliff, would be three unproven/unknown factors, Tim Duncan with knee issues, an aging Antonio McDyess, and Matt Bonner.
So what does Ratliff add to the frontcourt as it is besides age? Well, one thing that he brings that has been discussed before, is simply this, another shot blocking presence outside of Tim Duncan. Besides being a stop plug should one of Blair, Mahinmi, or Haislip not pan out, Ratliff is known first and foremost as someone who defends capably and blocks a lot of shots. If we look at his stats relative to the other veteran bigs (Bonner, Duncan, and McDyess) we see that Ratliff on a per-36 minute basis blocks more shots than even Duncan himself.
While youo're not going to get much offensive production out of Ratliff offensively, I think the Spurs generally have enough firepower to somewhat hide Ratliff on the offense, allowing him to gobble up offensive boards and put-backs. Besides, it's not like he's going to be seeing anything more than possibly 5-10 minutes of floortime per game. While Mahinmi's D-League numbers and Haislip's Euroleague numbers show some promise as being able shot blockers, having Ratliff as a contingency plan isn't a horrible solution.
It's possible that now that Ratliff is on board a trade is imminent as Rikkido has stated. I believe Rikkido's original proposal looked something like this: Micheal Finley and Matt Bonner for Raja Bell. Which makes some semblence of sense. As I've stated before, I'm also willing to part with Mason if what the Spurs are looking for is an above-average to elite level defender. While Richard Jefferson is supposed to be solid, adding another defender never hurt, and would provide a number of matchup problems as well as be a solid contingency should Malik Hairston and Marcus Williams not work out. While he's not elite, I wouldn't mind parting with either Bonner or Mason for someone like Damien Wilkins either. Of course at this point in time it's all just speculation, and won't really affect the team roster come crunch time, which I believe will still remain Parker, Ginobili, Jefferson, McDyess, and Duncan, however, should the team not be as healthy as anticipated or something of that ilk, then it's good to have these sort of contingency plans.