Should comments that have been made partially obsolete (e.g., by clarifications that were requested in the comments subsequently being incorporated into the question) be subject to deletion by moderators, if the comments also include relevant information to the final version of the question? (I ask because I just observed this occurence.)
Obsolescence can be hard to judge sometimes. We mods try not to delete still-relevant comments, but we might miss sometimes. This is particularly true if, as DoubleAA mentioned in a comment, a big long thread is being cleaned up. (Which usually comes to our attention via flags.)
Here's a scenario that has happened to me several times: big long comment thread with back-and-forth between the post author and somebody else, the post is edited, optionally the editor says "thanks I fixed it", and when I look at the revision I see edits that seem to be related to the comments. So I delete the obsolete comments. (If I really can't tell I'll leave the flag for another mod, but sometimes that just leads to nobody handling it.)
While all the moderators are reasonably knowledgable about the topic of our site, we're not all experts on everything (and I am but a grasshopper compared to the others). But that's ok, because moderators are not called upon to be adjudicators of correctness. If we miss something important, please bring it up -- but the best way to make sure important information isn't lost is to get it into the body of a post. Comments aren't meant to be permanent; important information should be in posts (questions or answers). If you have something that can help a question or answer and that doesn't go against the author's apparent intent, edit it in. If you have a major disagreement with an answer, write an alternative answer where you make your case.