From 2008 until now, a total of 124 baseball players played at least 200 innings in both center field and a corner outfield position. Here is the mean of their career UZR/150s at each position. UZR/150 is the best defensive rate stat; it tells us how many runs above or below average these players were at each position over the course of roughly a full season. Some players played at least 200 innings at all 3 spots, which is why the totals are a bit wacky.
(Aside: the flaw in these numbers is that a UZR/150 accumulated over several years at a position is weighted that same as one accumulated over 201 innings; if you have a better way of weighting it please let me know.)
The generally accepted positional adjustment for defensive difficulty (developed by Tom Tango) is -7.5 for left and right field and +2.5 for center field,which means we think there is about a 10 run difference in difficulty between playing center field and a corner outfield spot over approximately a full season. That means that when we are accounting for value, the centerfielder gets a 10 run bonus because the average center fielder is a much better defensive player than the average corner outfielder. That's close to what we see above-- we're looking at the same exact players who are just playing different positions, and there is about a 9 run difference between their observed defensive values in CF and LF.
Quite broadly there are three types of players who tend to show up most in the above sample. The first are corner outfielders who can manage to play center on occasion but aren't particularly good at it (otherwise they would be playing there full time). The second are defense-first outfielders who don't hit enough to be consistent starters, but fill in on the corners on occasion (the Logan Schafer role on this year's pre-Parra Brewers). Finally, there are aging centerfielders who move to a corner later in their career (for example, Ken Griffey Jr. is in the above sample).
All this circles us back around to Parra, who strikes me as a bit of an oddity. He's an extremely well regarded defensive corner outfielder by both analysts and scouts. However, it is rare that a player like Parra actually spends the majority of his time on the corners. Most players like him end up playing in center field, and the ones who don't tend to be those with plus range but a weak throwing arm, which we know isn't the case with him. Parra doesn't neatly fit in any of those 3 categories above. He's primarily played the corners, and spectacularly well, while being roughly average hitter. That's a very unique way to create value. His UZR/150 career averages, analogous to the small table above, are:
LF: 10.6 UZR/150 in 2559 innings
CF: 5.4 UZR/150 in 1046 innings
RF: 16.9 UZR/150 in 2434 innings
This is interesting for a few reasons. If we believe that Parra is a +5 CF and a +10 LF, there's an argument that his best role on a team is in center, because based on positional adjustments, center should be 10 runs harder. Obviously that's not an option right now in Milwaukee, and it's questionable how much we even believe that statement because if the comparison was right field one could conclude the opposite. But we can probably say at least that Parra doesn't seem to give up anything in center, which we might have expected based on how the Diamondbacks used him so far in his career.
The big takeaway here is first of all that Parra may be an ideal 4th outfielder for a team like the Brewers because he doesn't lose anything at any of the 3 outfield positions despite the fact that he's primarily a corner outfielder. Had Parra's numbers on both offense and defense not tanked in the first half of this year, he would have cost a heck of a lot more than the Brewers paid for him to acquire. With any reasonable bounceback towards the norm in the last 2 months of the season, having an asset like Parra will make the offseason more interesting because he gives the Brewers a ton of roster flexibility by having 4 capable starting outfielders for 3 spots. At the very least, he's a very unique player and the more I think about it, the more I like the move that Doug Melvin made to bring him in for such a small cost.