I think comments are important. They indicate that someone was motivated by what you wrote to the point that they were willing to leave a comment. That is a good thing. Whether the comment agrees with what you wrote or not makes no difference.
Now you owe the person that left the comment a response. At a minimum you should thank the commenter. Depending on the blog other readers may get as much or more from the comment interaction as they do from the original entry.
How you respond either encourages others to comment in the future or it may tell the readers to just keep their typing fingers to themselves. I think a good blog has a positive dialog with the readers of the blog in the comment area.
A well written comment can be as interesting as the original post. The fact that the comment may be the reader’s as opposed to the author is not important. A better blog will attract better readers and therefore better comments.