1. Jackson hasn't regularly been a good defensive OF, he's mostly adequate. UZR/150 by year
5.8
7.9
4.0
-4.5
-10
8.9
This isn't a gold glove type defense we're talking about here.
2. Jackson's inclusion to the roster would mean the Cubs FIRST have to trade Coghlan THEN hope Jackson signs here after that AND Coghlan can't bring back a 40 man roster type prospect
3. Jackson being happy getting 150 PA in a pillow year seems unlikely. It's a huge chemistry risk.
4. You keep quoting his career numbers when
A) Two of his best years came in the first three years and he's never approached that
B) He's never been in the NL so any adjustment time will impact his ability to meet those numbers
To point 1, i mean 10 UZR/150 is where you start talking about realistic names for gold glovers. So, obviously I'll concede he wasn't on the door steps of winning one but as I said before, how many above average(which 5+ UZR/150 would be) CF's are there year to year? Also not to nitpick but last year his UZR/150 was 10.1 which was 5th best in the league. Is he Heyward? Nope. Is he pretty good? Yes. So, I disagree he's "overrated" though that's obviously in the eye of the person rating him. He's not a great defender in the way Heyward is. There's orders of magnitude between those two but he's easily one of the 10 best defensive CF in baseball.
To point 2, I disagree. I wont dig to deep into this because I literally already went through it with TC. Simply put, my view is if your able to sign Jackson as a bench player he doesn't really have any say in how he's going to be played. I believe your case is he'd have to be the cubs 4th OF thus trade your present 4th OF. In my view, he's in a situation where he has no leverage and thus has no real choice. If he can get a starting gig or a 4th OF spot from someone else more power to him. But, if you're the cubs there's no way he's dictating terms to you and if he's going to be on the cubs he's going to have to buy into the idea of being a role player. If not so be it and wish him well else where.
To point 3, does he have a history of being bad in a club house? I'm legit asking that. I didn't follow him in detroit much so it is entirely possible he's been a me first guy but to this point I'm unaware of it if he has been. Without knowing more background it's kind of hard to discuss this. But I will say what player who's unsigned right now is going to be happy? The reality of the situation for those players is teams don't think they are good enough to give them the money the player likely thinks they are worth. That's just the way things go. And frankly, if it gets to the point where the cubs are going to sign him I'm certain they will have made 100% clear what his role would be and vetted that with him. If he's going to cause a stink about it, they flat out wouldn't sign him. But, unless you have an example of him throwing a stink previously that seems like speculation on your part that it might happen rather than something that is likely.
As to point 4, part A is 100% fair. He's not the 4 fWAR player of 2010 or the 5.4 fWAR player of 2012. But keep in mind the career numbers i'm citing are triple slash. He hit .273/.332/.398 with Detroit in the first half of 2014. He hit .272/.337/.417 in 2013 with detroit. He was absolute crap with Seattle for whatever reason hitting .229/.267/.260 in the second half of 2014 and more average in 2015 hitting .272/.312/.387. If you want to argue that's who he is now it's a valid view. I believe seattle is a barn fire of a team and he wasn't going to succeed there. You can also say he didn't hit well with the cubs last season at .236/.304/.375. Again a fair point. My view is it was only 79 PAs and wasn't exactly the most regular routine considering it was a playoff push. Either way, If we say he's the .267/.311/.385 hitter he was for all of last season with a 10 UZR/150 that's a pretty phenomenal player for what amounts the the 24th/25th guy on your roster. Obviously you'd prefer he didn't K as much but as I've said before what player in that position is perfect?
As for point 4 part b, do you have some historical data that suggests AL players struggle in the NL? I've legit never heard this argument before. I suppose I can see that changing some of your counting stats(RBI/R) but how exactly does that affect your triple slash? Only way I can think of is I suppose better hitting leads to more pitches thrown which leads to better PAs. But the argument feels flimsy to me. MLB triple slash average was .254/.317/.405 last season. NL triple slash average was .253/.316/.397 including the pitchers. AL triple slash average was .255/.318/.412. Obviously the AL was slightly better which is to be expected but it's very marginally. And if you look at NL triple slash average without the pitchers they are better at .260/.324/.410 I'm guessing because NL are more apt to platoon.