He didn't actually play that much in CF in the winter league at least in games. He played CF in the following games
Dec 29th
Dec 27th
Dec 23rd
Dec 18th
In fact, he played more 2B and I think SS than CF. Now from what I've read the cubs sent a coach to work with him so I'm sure he got some practice in. But it's a distortion of the truth to believe he got extensive game time in CF in the winter leagues. Again, I want to make it clear I'm not saying it's impossible for him to play CF. But as I said before with examples like Hanley in LF last year and Soriano before him it isn't always an easy transition from MI to OF and CF is harder than the LF those two both played.
I honestly just don't really see the down side to signing someone like Jackson if you get him for a bench salary. Obviously that's the crux of the issue and if he gets more money you're not signing him. However, if you get him for say $3 mil how do you lose? If you want to play Baez in CF over him because Baez looks amazing in ST then fine. That option is still there. At that point it's only costing you the money to keep him around. But again, to me it's not Baez vs Jackson as to who plays. It's Jackson or La Stella. As I said before, I think Baez is going to get his PAs either way.
La Stella is fine as org. depth but he honestly doesn't do much for me. He's got basically 0 power(2 HRs in 435 PAs). He's been a below average defender(-1.7 UZR/150 at 2B over 809.2 innings). He doesn't offer you any real speed for pinch running. He hits OK but it's nothing really to write home about. His RHP splits are basically dead on average at .246/.318/.318. His LHP splits are decent at .292/.370/.400. Jackson is a career .261/.336/.393 hitter vs LHP and .278/.331/.402 vs RHP. Jackson has regularly stolen 20 bases. He's regularly been an above average defender in CF. And while Jackson doesn't have great power, he's good for around 10 HRs a season. Plus, it's sort of easy to forget how young the cubs team is. Jackson would be another vet who's had deep playoff experience with detroit going to the ALCS in 2011, the WS in 2012, and the ALCS again in 2013.
Edit: one last thing. If you're feeling is just "who fucking cares?" I get that too. I mean we're talking about likely the 25th guy on the roster. It's not exactly debating the merits of signing Heyward. To me it's just A) there's nothing else left to talk about at this point and B) if the cubs are going to fail it's going to be lack of depth in vulnerable areas that gets them. I mean I'd honestly be fine if they gave Jackson $3 mil to just be a glove in CF and he didn't get more than 100 PAs. That way if the worst does happen you have options. You're not solely counting on Baez. And again, maybe Baez is great in CF but having redundancy isn't a bad thing.