Mills is not a bad player. He's actually pretty solid and I don't have a problem with him being on the roster. In fact, I wish he could get more minutes. But he's not a good point guard. That much was really clear in the chances he's had to play as one for us. He doesn't pass very well and he's just not much of a playmaker as far as distributing the ball goes. What he is, is a change of pace kind of guard who can bring energy and provide a boost with his shooting.
What the Spurs are looking for right now is a guy who can distribute the ball, get the team into its sets, and by extension relieve Manu of some of the playmaking responsibilities of the second unit. Minus Neal, out of the 3 guards we have left (Joseph, De Colo, and Mills) De Colo and Joseph are pretty much the only 2 true points. Unless Mills suddenly learns how to balance his own scoring versus setting up his team, having him run the point would just be more of the same problems we're all complaining about now with Neal. Albeit with better ball-handling.
It would backfire spectacularly in the playoffs unless he was hot the whole time. Kind of like what's going to happen if Neal keeps backing up TP.