I do acknowledge Suns are more than capable of playing defense, as witnessed by their decent defensive numbers at the start of both last and this season, it's just that by playing a short rotation, it is extremely difficult for the team to excel at both ends of the court for the whole season, especially when you have to cover for Nash and Stoudemire (more on this later). When push comes to shove, we all know which side of the court D'antoni will choose to concentrate on.
Speaking of Stoudemire, he is probably the worst defender in the league given his athleticism. I cannot understand it, he is strong, quick, coordinated, has great hands, can jump out of the gym, and yet he cannot understand a thing about boxing out, defensive positions and rotations. It is frustrating watching him play, as he has all the physical tools to be the next David Robinson on defense, but he just turned out to be a tool on defense.
Skinner is full of enthusiasm, and I would love to have some guy like that on the Spurs (man, I would trade Cisco for him in a heartbeat), but he is not as physical with Duncan, and we have known the only way to slow down Duncan is to rough him up, make him expend energy and tire him out as the series goes on. Skinner just isn't that good at that part of the game.
Finally, I agree with the Hill assessment, his mid-range game will present problems for the Spurs, as the whole Spurs defense is geared towards making other teams make long 2s (reason why the Mavs pose such a problem for the Spurs). Stoudemire and Nash both have a good midrange shot (great for Nash), and by adding Hill, there will be 3 players that the Spurs would have to adjust their defense around. However, all three are also terrible defenders, and the Spurs will get theirs. look at last year's playoffs, outside of Games 2 and 5, the Spurs have scored an unusually high number of points (Game 2 the Spurs couldn't throw the ball in the ocean, Game 5, the absence of Stoudemire took a huge part of that in-between game of the Suns, and forced both teams into a defensive battle).