Ok, i get it, people are just to lazy to do that work (but they find the time to integrate disqus...) but from my point of view they are leading me to a website i don't want to use at all. Disqus is like a hidden website embedded in the one i want to visit. The difference is that I know that and i block that. Other people use that service and might not even recognize that they are leaving their tracks on some other website they don't even know about. And in contrast to the usual ad/cookie tracking they might even leave their email address there.
And seriously... an easy to setup CMS, that has a "user identity system, optional openID, an anti spam measure like recaptcha"? Does such a feature-beast even exist amongst the quadrillion CMS that exist since the dawn of time? (<-- sarcasm)
When i go to blogger or tumblr or wordpress and leave a comment i am aware that i leave a comment on that site, and only on that site. That's a big difference. To me at least.
From my point of view the hoster of a website is somewhat responsible for my user data. He shall not send my password to some other party, or he shall not send my email to some other party, etc. Using disqus only shows that he doesn't really care to the extent i would wish my data to be handled.
Of course it is up to me to use disqus, but, as i mentioned, 99% of people wouldn't know what is happening.