I agree with this statement. Mooney likes guys in back-up roles who "know" his system and don't mess up. That is largely Grace, particularly on offense. He passes well, doesn't shoot unless it is a wide open look and doesn't turn it over. All good things in Mooney's vision of a back up center, particularly when we can surround him with 4 good scoring options. .
However, if you watch Grace on defense, you will see that despite his size, other teams will immediately attack him in the interior, backing him down in the low post, often times with success. His lack a level of aggressiveness and perhaps strengths makes him exploitable on defense.
I haven't seen Weir in a college game, but he appears to be a big strong kid. If Weir wants minutes, he makes his impact on defense, because I do agree that Grace is giving Mooney, what Mooney wants on offense