Like many here, I turned off commenting years ago. My motivation had less to do with spam. (I'm on WordPress and it does a pretty good job with that.) It had more to do with the conversations moving from my site to the social web. Folks wanted to talk about stuff where they already were, rather than centralizing that conversation on individual blogs.
(As an aside, I rarely participated in those comment threads on my blog. I always saw them as a place for others to talk about a post. I'd already had my chance to say my piece. So the comments had less value to me, personally, but seemed to have value to the folks reading my blog. All to say that I wasn't really driving engagement with the comments. I was just letting them do what they were doing.)
I tried a few of those "commenting" solutions that attempted to pull the social channels into the blog itself, but they never seemed to recreate that 2005-2010 dynamic of active and engaging conversations occurring on a single post.
It's also worth noting that, in my experience, a big motivator for many platforms had to do with driving pageviews. And once commenting stopped doing that, folks seemed to lose interest in continuing to offer that functionality.