When teaching I am always telling my students that buying a lens filter for the sake of having a filter is not a good thing, the only thing that you are doing is adding to the stores profit margin. Sure I have heard all the sales talk about how a lens filter will protect the lens glass and save the lens, but honestly in all my time taking pictures and teaching I have never seen a filter that has protected a lens, or for that matter a lens that had the front glass scratched. It may happen but I have not seen it.
The other night when teaching Astro photography one of my students took a picture that I was shocked to see all the reflections on the image. So I asked him to take the filter off and take another picture. Sure enough the difference was amazing!!
Even the best filter can cause your lens to loose quality and the two pictures above show just one thing that can happen. With the added glass you can get softer images, lens reflections, etc.
If you have a real use for a filter buy one, if you are just buying because the sales person has told you to, be careful. I strongly suggest that if you are looking for a filter, or filters, look at Lee or Cokin filters. They are not designed to be left on the lens but do a great job of “filtering” your image.