Ben Zobrist's UZR/150 at second base is 25.9. He's only played 400 innings there in his career, and ideally we'd have a few thousand innings to judge. Which means it's still far too early in Zobrist's career as a keystone member to assign that level as a reflection of his true talent.
Zobrist played a little over 1,000 innings at shortstop and rated at -8.5 UZR. 6.1 of those negative runs were from errors, either booting balls, under/overthrowing the bag, whatever. Zobrist's error rate is actually in the plus now, and his range is through the roof. He's split playing time between the corner outfield and second base. While transitioning a career shortstop you would expect him to gain 5 runs defensively moving to second and 15 moving to right/left.
It's hard to tell whether Zobrist's error rate was true talent or simply a blip, so if you assume he was closer to -5 than -10, your second base expectations would be a completely average second baseman. Right now, he's on pace to be the best defensive basemen since 2002 by an entire 6 runs. That's highly, highly unlikely to happen.
So, if we only have 400 innings instead of a few thousand, how should we project his defense moving forward? Well, by staying conservative. His age certainly does him no favors either, as range starts to decline after age 24. Zobrist is well past that point and is putting up absurd range rates. Even assuming that Zobrist is safely a +10 defender is hard to do. Let's look at the amount who qualified at +10 over the last few seasons.
2008: 4
2007: 4
2006: 2
2005: 5
There's ~35 second basemen a year over four years, or close to 140 cases. Only 15 finished with UZR over 10. That's not encouraging. Maybe Zobrist is a true talent 10+ defender at second, but I wouldn't put money on it based off 400 innings.
I would keep expectations in the 0 < x < 5 (10 if you're feeling dangerous) range.
Of course, everything else that conventional wisdom said about Zobrist has been wrong. This is the one player I may as well throw my arms up about. What's he going to do next? Beats me.