In the last three years, he's -1.1, -4.1 and 0.4
He's 32. He should be done playing CF. In all honesty, I see him moving to LF in the next couple years. Hopefully, by then, Harper will be able to play RF and we will have found a good CFer.
You conveniently leave out the year he played his most innings in CF, which was 2008, when he had a 5.1 UZR. If you look at years he played over 100 innings in CF, he has had a positive or neutral UZR in 3 out of 4.
It is silly to say the numbers prove he is a bad CF justt as it is silly to say that the numbers prove he is a good CF. He simply has not played enough CF in the majors to say that the numbers show he is good or bad out there. At this point, one has to more or less infer his ability based on judgment, eyes, body, athleticism - all those sort of "scouting" techniques. If you want to say anything about the numbers, then you can say it looks like he has had good range and arm numbers throughout his careeer if you look at OF totals. 2010 in Philly looks like an outlier and I can't tell you why. This year, it seems he had more trouble adjusting to the OF wall in right than in playing center.
Since I've never seen a right fielder in Nats Park play the video board / fence portion of RF well, I assume that between the height, light, and the grating / fence over the video board, I'm guessing that that wall has an effect on peripheral vision and affects defenders, so I'm taking the slightly negative range numbers in right with a grain of salt.
What Werth is right now and maybe for the next couple of years is a good range, good arm outfielder. I don't want him out in CF when he's 35, maybe, but for a couple of years, he should be a viable option. Maybe the same is true of Harrper, who has the tools. That could leave Rendon left, if not 2d or 3d.