RC takes risks on draft night. He goes for developmental no-name players. Sometimes he comes through and sometimes he strikes out.
Right now Mahinmi looks like a questionable pick, but this kid has the potential to be a beast. If he turns into a beast in two years will you be questioning RC then?
And what about Splitter? How was that a bad pick?
To be honest, that's not even the point...
The real point is that RC is allowed to take those risks on draft night because he has built a championship-contending team year in and year out by piecing veteran role players with his core and not by relying on draft night selections/rookies to fill critical roles. Guys like Bowen and Horry will be gone in a couple years and the ones that come in behind them (Udoka, Bonner?) won't be green rookies or flashy superstars...they'll be quality role players.
Bottom Line: If RC thinks Mahinmi could be the next Robinson and Williams the next Elliott, why not take a chance on them and see if they pan out? In the meantime, the Spurs will continue to build their championship-calibur teams without having to count on that coming true.