Why must I have a "strong" password for sites like this?

I don't share any personal information with StackExchange, I'm not really worried about anyone trying to hack my account and I can't see any incentive for them to do so, and yet the password strength requirements are about the strongest I've ever seen.

Why does this site, and others, insist on strong passwords? What's the reasoning behind it? I can understand why my bank might insist on high security, but isn't it up to me how secure I want to be on here?