This seems like a very weird way to look at the issue.

For one, not being able to understand minute, uncountable connections and interactions doesn't mean we can't realize a broader relationship of causality between them and our own actions. There are many things we don't know - that's right and undeniable - but there are also many things we do know, or at least that we think we know. Sure, you can go around saying "we understand so little about [virtually any scientific discipline], might as well assume that whatever soothes my psyche is true," but just because the first part of that statement is true doesn't mean the whole thing is reasonable. In my opinion, by the way, it isn't reasonable.

Assume free will exists; if you are wrong, it will made no difference;

Here's a question for you: if you assume free will doesn't exist, what difference does it make? I mean, you still feel like it exists, you live your life as if experiencing it, and regardless of whether you, as an individual, believe it or not, the world continues on as if it does exist. I really see no difference, in practical terms, between believing free will exists or not.

A little off-topic, but this reminds me of those people that say that morality can't exist outside of religion. You say you're an atheist, and then they ask you why you don't go around killing people. Hopefully you understand what I'm talking about here.