The thing with wood is it sort of depends how they plan to attack a #5 starter. Fans wouldn't be happy but you could conceivably run Turner, Wood and one or two other names out there for your #5 starter. And worst case, you end up with Wood being a lefty long reliever which has more value than a righy in a similar fashion. $6.4 mil really isn't that much money when you're talking FA prices. For example, Jason Motte got $4.5 mil and Zach Duke got 3 year $15 mil. That's obviously just looking at him as a RP. Aaron Harang got $5 mil and Kyle Kendrick got $5.5 mil as starters/#6 guys.
You just simply can't have enough pitching depth these days. You could make a good argument about going 6-8 pitchers deep with some in Iowa. We've seen Wada get play this year with Richard, Beeler, Roach, and Harren all seeing starts. Jackson also played that mop up role you have to have on every team. On the plus side, the cubs probably will have Johnson in AAA next year as on "free" arm. I imagine they will work Edwards out of the pen mostly though a spot start here or there wouldn't be crazy. Carlos Pimentel pitched well and again isn't a terrible depth option. Jokisch is also around. And finally Turner will be around most likely so that gives you around 8 or so starters but going into spring training you probably want another 1-2 more.
Regardless, at $6.4 mil I don't think Wood is just a liability that you let him walk unless you have definite plans to upgrade. He's "the evil you know" and has a 2.95/2.53 ERA/FIP out of the pen and has been tied for the 2nd most valuable reliever(just looking at reliever stats here) with 1.1 fWAR behind Rodon and tied with Strop. And you're not paying him for a long term commitment. The risk with FA is you don't always know the guys you're getting and how they fit into the club house. Presumably Wood does fit as he's still here. So, sure he's probably over payed by $2-3 mil making that much but I think you can make a rather compelling argument that not rocking the ship is worth that. Obviously if that $2-3 mil is the difference between acquiring say David Price or whomever top flight starter then sure I can see that argument. However, I'm not sure how quick the cubs are going to be to play market price on another starter. I think they will try to trade for someone before signing one because they have a number of guys they can't protect from the rule 5 draft anyways.