Reasons I Keep Comments Open
While I can understand why closing comments on older posts is a good idea due to all of the heavy spamming that goes on (I have considered doing it myself from time to time) ultimately I think that commenting should stay open for the following reasons.
One major goal as a blogger is to receive comments on their posts. The last thing you want to do is to have a first time visitor to your site come in on an old post, get really excited about responding to it, just to find the comments are off. Assuming they are not the ultra determined type to go seeking out your comment form or other contact information, they may just leave and you will not get the chance to interact with them and possibly convert them into a loyal reader.
Bloggers strive to create content that is timeless. Sure, there is lots of content that will go out of date due to new technologies (like posts about the features of WordPress 2.7, soon to be obsolete thanks to 2.9). But there is also content that will be relevant now, several months from now, two years from now, and so on. If someone finds a post that means something to them, I want them to feel welcome to comment on it, however far into the future that may be.
Reminder to Update Old Content
If you get a comment on an older, successful post that might be a bit out of date, it might be a nice reminder to update it and transform it into a new article. Chances are, you have gained new readership since that original post which will make your new article even more successful than the first. You can even capitalize on the success of the original by updating it with the link to the new post.
Search Engine Benefits
Search engines like pages that have new content. New comments update old blog posts, and that probably helps bump them up in search engine results. If you bring in a new reader through search engines, even if it is on an old post, they may like your writing so much that they will move to subscribing to your new stuff.
If you have an older, popular post that a lot of people have subscribed to the comments via email, all of those readers will get a friendly reminder about your blog thanks to that new comment. I have received email alerts about new comments on posts I commented on well over a year ago, and thanks to that reminder, I went back to their site to see what new topics they were covering.
Motivation to Write
Maybe a blogger has found themselves in a rut and feels like they have no great new content to write about. Getting a good comment on an old post may be a nice reminder to them that they have written content in the past that still brings people in to their site. It could be the inspiration that leads them to writing new content.
Handling Blog Comment Spam
So how do you handle all of the spam from older comments?
In WordPress, you can set moderation for every comment, or let comments be approved automatically after a certain number of comments by the same author have been previously approved. Although you do have to keep an eye out for those commenters that leave a few awesome comments, only to be followed up by spammy ones.
Although I have had my issues with Akismet, the WordPress plugin for filtering out spam comments, I ultimately believe it is more beneficial because it keeps a lot of spam from ever hitting your site and email box (assuming you have alerts from WordPress every time you have a new comment in moderation).
Do you have comments closed on your site after a certain date, or do you allow comments indefinitely? What are the pros and cons you have experienced?