How many times has Pop relied on the bench to get the Spurs back in the game? This season, we've seen Pop do something that we've rarely seen him do in the past: he rides the hot hand. If Mills comes in hot, he usually gets kept on for a few more minutes. His superb performances against the Clippers and the Blazers will surely give Pop the confidence to use him more in the playoffs.
You can say that Mills is horrible on defense, but that doesn't change the fact that Parker is just as bad, if not worse, than Mills on the defensive end. Mills has a better defensive rating than Parker, and most posters on here would agree that he puts a lot more effort into fighting through screens than Parker does. Parker's definitely going to struggle to defend Dragic, Westbrook, Lillard, Curry, Reggie Jackson, James Harden, Jeremy Lin, and pretty much all of the other guards from play-off teams, but that's not going to stop him from playing big minutes. Last thing to factor in is Parker's health. Will his body withstand 38+ minutes of playing time? He hasn't exactly been injury-free, and Pop knows better than to play him all out every game even if it's the playoffs.
Danny Green wasn't just benched because Pop was 'tinkering with rotations'. He was benched because of defensive mistakes and because of his inconsistent shooting. Some nights he'd be 1/9, and the Spurs just went with Marco to finish the game.