Defense presuming you're talking fangraphs is position relative. In other words, that number is comparing him to other LF not OF as a whole. When Cogs has played CF with the marlins his UZR/150 was -15.2 over 800 or so innings. You don't really want him playing CF.
I was looking at that also. Cogs hasn't played CF sense 2013. That year he put up a -17.9 CF in 150 innings. LF -9.6 RF: -35.4. Over all -16.3 OF. This year: LF: 17.8. RF: 10.9 so not even the same guy.
To give common ground Soler in RF: -14.0 I think it shows that a guy can vastly improve. But it also shows that Coglands D is a positive value this year and the view points are unjustified.
Just as another point Fowlers UZR150 is -1.3. So again a dislike value.
Again I see little info to back a opinion that Coghlans is a liability. If anything it points towards an improvement at all 3 OF positions.
Here is the thing. I believe Coghland is a starter on this team. I believe he could start in CF and hold the fort down with out losing much. His OBA is at .341 with a .282 BABIP VS Fowler's .349 with a .309 BABIP. So a .020 higher luck value with a .006 OBA increase. Kinda points towards a better player potentially.
I think it is a reasonable option. They could even re-up with Denorfia to continue the platoon. His D is better. They are proven. Both have lead off skills and it becomes a reasonably priced option in CF. Not to mention both bring OBA ability.
It is not like it has not worked.