Actually, at first I'm mad at them because they don't track back, even Messi, but I realise that we always ask for more, as fans. But then I saw them against Bayern defending a 2-0, with us having 10 field players behind the ball (even Messi), so it can be done if the game needs it and if they are motivated enough to do it. And with Bayern attacking us to get the away goal, we really needed them to do some defensive work. And they did. They didn't do it when Juve was gaining momentum at 1-1 in the CL final, so there are moments and moments. That 10 minutes between Morata's goal and Suarez' goal were the only moments when we didn't know if we'll win or lose the final. Anyways, the idea IMO, is that we can't really ask them to track back or count on them to do it.
Now, the part where we don't agree is the creativity of the midfield part. I think our midfield was basically part of our defensive mecanism (and I don't mean defending by keeping the ball, but about defending when the opponent has it), more so than in the other years. If we look at our defensive record this last season, we might say Pique stepped it up and Alves got back to his best, but even so, our defense was this good because our midfielders were much more preocupied with the defensive aspect of our game. Right now, if we are to play Thiago Alcantara instead of Rakitic (just an example), that would result in a loss in our capacity to 1) get the ball back in midfield, 2) tackle an opponent or make a tactical foul, 3) cover for Dani Alves and others. These 3 things are much more important right now in our team than that extra bit of creativity that Thiago can provide and Rakitic can't.
That is the reason I like Turan and I think he's an awesome signing. In a Barca 2015/16 set-up, Arda Turan is basically a ball-winner with flair and skill for me, that could be more if needed (very important) -- even a playmaker, assist maker or scorer. Even if he played in different positions, and he might not be 100% CM, his qualities check all the marks for a Barcelona CM at THIS moment in time. Maybe I'm overrating him a bit, but if we're happy with Rakitic, I think we'll be even happier with Arda.