Recently, I was speaking to a group of CPA firm marketing directors about blogging. During the Q&A, someone asked "Should you ever reuse content?" My answer was "welllll ... it depends."
If you bring back an old post because the information is still relevant and useful to your audience, that's okay. I've done it as have many others. Just note that it's a reintroduction of a past post.
However, if you reuse posts because you either don't have the time or the energy or the proper respect for your audience, then it's a very bad idea. I removed a blog from my RSS because I began to see the same posts again and again. This blogger is very respected in his field but by recycling old posts, it felt like the blog was just a cynical marketing device rather than an attempt to create a dialogue with readers. The best blogs create that dialogue, that community, and the most avid readers. As such, they are a very effective marketing tool. By cynically recycling posts, you are diminishing the marketing effectiveness of your blog.
My answer, I think, rang a little hollow because of the touchy-feely nature of it. In order to give them something to hold on to, I gave the example of an old pastor of mine. As he neared retirement, he started telling the same old stories in his sermons. You could tell he was phoning it in. Not good for pastors and not good for bloggers.
So, the moral of the story - don't phone it in. If you are considering starting a blog for you or your firm, know that it is a commitment of time and, as a rule, the content should be fresh. It is okay to repeat a post once and a while but only if you note that it is a repost and only if it is information that your audience needs.