Commenting is something I'd much rather offload to someone like DISQUS. Creating your own commenting system either requires lots of spam management (for an open commenting system) or forcing users to register for your service before they can comment.
Frankly I prefer to use my existing identities to comment than have to sign up each time on someones service just to be able to comment.
I prefer just the opposite: I like having my identities on each site distinct and autonomous. If everyone has a common identity across multiple sites, it prevents any particular site from effectively evolving its own internal community and cultural norms, and raises the stakes of participation.
I segment my identities. A very few things will get my real name (or some variant of it). Most go under a generally-topical alias of some sort or another. I don't mind those aliases gathering their own reputation, but it's no major loss if I decide to toss one at a later point.
I guess I don't really understand this, because I see just as much - if not more - spam in DISQUS discussions. Making it easier to moderate is nice, but someone needs to actively bother to do so.
Frankly I prefer to use my existing identities to comment than have to sign up each time on someones service just to be able to comment.