There's something about the word "sync" that implies, y'know, a sync. I said no to contact sync when I installed Facebook for iPhone and that was that -- no problem.
And that is what I find most objectionable about Facebook's user experience in general.
Everything I do, every click, I have to weigh: What will this mean for my privacy? Will I accidentally share something I didn't want to? I have to think everything through from Facebook's perspective and try to predict what my actions will be interpreted as consent for the system to do. It's decidedly unpleasant.
Not if it's blocked.