Steals and Blocks are the canary in the coal mine of defensive stats. If a player is consistently logging these stats, they are indicators that they are putting forth at least some efforts at some of the other invisible stats. A player that blocks 2 shots a game, for example, has probably altered several others.
In addition, Anderson is a good defensive rebounder. A defensive rebound would have won us another championship the first time around with the Heat.
That stuff is all real defense.
Duncan blocked a lot of shots and altered a lot of shots, and changed the way offenses schemed because of it...
No, it's not on the ball defense, but it is defense, relevant, and helpful.
That being said, hopefully he can improve his on the ball D at least a little as well.