I don't think it's so much that Jones is declining, I think he had 1 bad year. I've already showed the comparison between Jones and Rowand at Petco Park, Jones having the upper hand in a big way.
Jones' numbers have declined over the past 3 seasons, but I still think he's one of the best defensive center fielders in baseball, and still has one of the most dangerous bats in baseball as well.
Let's look at the comparison's between the 2 though, this might give us a better look at it.
Jones/Rowand
Age: Jones 30 Rowand 30
Seasons: Jones 12 Rowand 7
Gold Gloves: Jones 10 Rowand 1
100 RBI seasons: Jones 5 Rowand 0
20+ HR seasons: Jones 10 Rowand 2