Commenters can log in via twitter, Facebook, Google+, and others but if they already have an account on your site, they can't just use their credentials from that to post. It doesn't make sense to me.
That question applies to Disqus (the service) and the short answer is that, being a web service, it's not meant to integrate that deeply into Joomla!. E.g. it's got benefits (like the anti-spam features) which could never be implemented into a Joomla! commenting system.
The support is horrible though. A number of my clients have upgraded to 2.5 and the 1.5 version of this component is no longer compatible. To get a updated version of this component, though, you have to pay again. I understand paying for upgraded features but when the core system itself requires the component to be updated, requiring payment again is just taking advantage of the customer. Especially when the new version of the component is more of a update rather than a upgrade to make it compatible with the current Joomla software. None of my other commercial components or modules required duplicate payments for up to date software. I'll be searching elsewhere for other solutions from now on