Not losing much? A 22-year-old C with quickness and athletic ability isn't a lot? Not to mention the fact that, for his age, he's actually quite developed, especially when you see the other Cs his age that are highly thought of (Thabeet, for example) and have similar talents. I understand what you're saying that Sefolosha still has potential, along with far more polish than Mahinmi. However, Mahinmi, despite what you may think, has a TON of potential (more than Sefolosha). The only thing that really stopped him from competing for a spot last year was a badly mis-diagnosed injury. Why would you give up on Mahinmi when he hasn't had a real chance to prove himself?
Not to mention the thought that even if Sefolosha had the same amount of potential that Mahinmi has with his current level of polish that:
Young, very athletic C > Young, talented, defensive wing
Why? Simply put, there are a lot more talented defensive wings out there than highly athletic centers. , why not give Hairston the chance to be that defensive wing that we all covet so much? If he doesn't turn out to be that by the trade deadline, then you can make a trade for a defensive wing, and you might not have to give up as much to get him (depending if the trading partner is out of contention and desperately trying to free up cap room).
All that being said, there is absolutely ZERO reason to trade Mahinmi at the moment, and it would take Scola trade-esque stupidity for the FO to do so.
Add to that, he may turn out to be a better option at back-up C than Ratliff this season; it wouldn't shock me in the least if Mahinmi out performed the 36-year-old who only played in 46 games last year AND who has a rather long history of injury.
Oh, and also, game creators use past performances to decide the ratings of the players, so a video game is not a good indicator of future successes/failings at all, especially during the off-season.