On its own, the information in those private accounts is probably not interesting. I used to use the "few sites get a unique and secret password, but most reuse the same 10 character one" ruleset, but I became worried about how much data could be aggregated about me. By re-using the same password, I felt like I gave a simple test that attackers could use to definitively confirm "user XYZ on site ABC is the same as ccooffee".

I'm now firmly in the "everything gets a unique password" camp. There are 4 important passwords I type myself, but everything else is in a password vault.