Ariza is a strong athlete and a quality man defender he does however lack offensive effect outside of putbacks and dunks, so he didn't fit the bill for the Magic...as for your statement, how many games has he played this season? how many games has he played in the last 2 seasons? he has injury problems, that foot injury he has is an ongoing issue for him, it started in NY and has followed him to LA where it has now cost him nearly the entire season...sad, because I was a big Ariza fan...
as for the trade, speaking on this season alone the Magic won that deal hands down, it was the perfect trade for the Magic...Evans is basically an evolved Ariza, he's very crafty both on and off the ball, posses strong defense and is a cleanup guy and Cook is shooter, which is what the Magic wanted...
so...i dont really see how keeping Ariza would have any bearing on the Magic this season, or this Spurs game...