And you owe it to your readers to stay in touch with recent trends. They'll keep reading for sure. I tend to think of it as a means to an end, rather than the end in itself. This of course requires knowing who your readers are, or at least making an educated guess. Chances are, they're interested in what you told them you'd provide them in each issue when they subbed up.
Usefulness is an interesting thing.
If it is original to that person, it stands a much greater chance of being something they can use. But originality doesn't imply usefulness. If you're writing your own articles, but are simply rehashing stuff that's already been consumed by your readers, it won't be useful. Ask yourself if what you're producing is worthwhile. Also, unoriginality doesn't imply non-usefulness.