This is really coming at the right time for me. Earlier today, I discussed this issue with my brother @dorth. People spam the comment section in hopes that they might get an Upvote if the author of the post sees their comments on time. Some authors have however proven them right but upvoting commenters on first-come-first-serve basis.
Many meaningful comments are usually pushed down out of sight or lost in the junks. One of the ways out of this is for writers, especially those with high voting powers to take time to read their comments and upvote rewarding ones or they should just stop upvoting and see who really wants to sincerely respond to the content of the post.
The issue of self voting comments for visibility is another prevalent issue, but I guess that might not change, even though it is not encouraged. I'm glad you pointed this out. According to @dorth, steemcleaners cannot do it alone. If each person decides to clean his posts like you have decided to do, then we can restore some sanity into the community.
I agree completely re: a lot of good comments getting pushed down. The new comment boosting option from MinnowBooster is not going to help in the comment spam regard, but that's another topic for another day.
One thing I will point out though...if you think it's bad now, wait until the platform grows, with 1, 2, even 10 million users. What will the comment spam look like then? It's a problem whicih won't go away.
I think it'll need addressing in the same way traditional (pre-Steemit) bloggers have done - and many turned off comments altogether.