Whether or not you'd provide your login details, the business model is far from dumb. They have an amazing amount of information about you that can be used for incredibly targeted advertising.
as to the whole security issue -- is anyone scared about making online purchases these days, like they used to be a few years ago? mint is as trustworthy as anyone else in the online finances business: http://mint.com/safe.html
I've seen environments from the inside out where credit card and social security data was treated like a joke (HIPAA be damned). The weakest link in any environment isn't the hacker path, it's the disgruntled employee. When it comes to my bank data, the less eyeballs in the mix, the better.