For people running home brew blog software or people running their own wordpress installs, etc, it may offer a nice decentralized way to avoid spam as well.
I'm not saying it's worth millions, but it doesn't seem entirely useless.
It's not just blogs. It's also new web apps, aggregators, wikis, mashups, and documentation.
Users also get to reuse their avatar/username across multiple sites which might encourage commenting.
It has good nested threading support which most blog software doesn't include for some psychotic reason.
It has good anti-spam which prevents you from needing akismet or defensio integration.
They support video comments now that most blogs won't support.
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Currently, I'm debating adding it to bug.gd rather than adding new features to our own commenting system. Why reinvent the wheel if we have a tool that does it for us?
The only cons I see right away:
Loss of serch engine benefits of having the comment content hosted on someone else's site.
Risk to your site if it goes down.
I also wonder how it handles incredible traffic.
You have answered your own question. If you don't use "every major blogging platform" that is where Discus comes into it's own. Users want the ability to have commenting but maybe not hosting them. You still get access & own you own data sans the problems & limitations of homegrown or open source blogging software.
How could they make money besides advertising? The WordPress guys or the Six Apart guys might pay for spam-free comments.