Mahinmi is playing well for Indy. Good solid contributor off the bench.
And an important one at that because Mahinmi provides legitimate size and defensive presence when Roy Hibbert goes to the bench. Hibbert is not a high minute player. He plays less than Garnett. He is usually in the low end of that 28-30 minute per game range.
Plus, Indiana has two undersized PFs in David West and Tyler Hansbrough, neither of whom provides any real rim protection defensively, so there is a big need there for Indiana to have a defensive orientated center alongside them at all times. And with Hibbert minutes limited, that means they need a quality backup center.
So, because of those reasons, Mahinmi is a valuable contributor for Indiana. He balances their team. Important glue guy off the bench. If Indiana were to part with Mahinmi, they'd need a quality replacement center coming back to them. He is too valuable to give up otherwise (unless getting a large return for him).
Furthermore, and finally, Indiana gave up Darren Collison to get Mahinmi there in the summer so I'd say they value Mahinmi pretty highly. That Mahinmi would be difficult to pry away from the team. Very expensive (relative to his talent) to acquire.