You never know when something you wrote about almost a year ago becomes the latest hot topic http://www.learntoearn101.net/2009/04/never-ever-ever-delete-old-post.html
Funny story. This does follow the line of thinking of writing about something brand new before it hits the news or writing about a talk-shows next day's subject.
You never know when someone else is going to be interested in the same stuff that you were interested in last year or the year before....
I guess if the information becomes outdated and you want to keep your blog up to date then you may want to..... If you have a post talking about this "new" technology coming out that lets you bank online then you may want to update it
It doesn't cost any more money to keep an old post around. You never know when a term of phrase in that post will get you mysterious traffic one week. Happens to me all the time...I still get traffic from a couple of posts from my first days of blogging.
I've done some paid posting. And every now and then I think about deleting an old one. The TOS for the paid posting I do only requires me to keep the post for 30 days. And I figure after a bunch of months, the advertiser has gotten their money's worth But then something like this happens.
Also, having a lot of indexed pages can help with building links as well. But, old posts don't turn old until they are 3 years out. lol I've had some posts from 2005 start taking off and generating traffic for the rest of the site, you just never know.