I saw a lot of defensive improvement from 2014 to 2015, and his UZR/150 went from -3.7 to +0.9. I was very happy that it appeared he was going to become a plus defender over the long run.
However, last year he looked ho-hum. He wasn't bad, but it seemed to me, he looked like he took a baby step backwards. His UZR/150 went down to -2.8.
He Defensive Runs Saved went from -9 to -1 to -10 over the last 3 seasons.
Inside Edge shows that although he made more "unlikely plays" than ever before (29%), his "likely plays" made went from 89% to 64% over the last 2 years.
There's still plenty of time to turn it back around and get better on defense, but I doubt he ever gets to top 6 or 7. With his bat, top 15 would be good enough.
The whole moving Bogey argument is moot right now, because we have no clearly better defensive SS other than Marrero, and he does not belong on the 25 man roster, let alone starting everyday. If we acquired a decent hitting and great fielding SS, then let's talk. Marco is not a plus defender at SS and neither is Holt.