why? because he is great there. He comes in and plays his ass off for 15-20 mpg and we get 110% out of him.
Physically I think he will wear down with the more minutes he plays and the numbers and efficiency will drop off. It's the classic case of "4th Outfielder Syndrome" as I like to call it. Every baseball team has that 4th OF who comes off the bench or plays a few days makes a ton of hustle plays and is hitting over .300 so everyone wants to start the guy. Those guys wind up starting and they get over 300abs and all of a sudden the great plays are less frequent and the batting average is around .250. It's ok to be a great roll player.
Perhaps next year with a full season under his belt, a stronger knee, a more toned physique he might be able to handle more of a load. Let him continue to be the great spark off the bench. If he is the guy in there at the end of the game I have no problem with that but don't start him.