a better way to protect your lens is a lenshood, it will improve the image quality in many situations.
i never use protective filters. although all my lenses don't show any marks after years of cleaning them with my t-shirt i do not really care about the super cleanliness of my front elements. i strongly believe that it doesn't make any difference.
just for fun, do this little test: take a small piece of a post-it note, the size of a pea. look through your lens, then stick the paper onto the front element and look again. do you see it? i doubt. if such a big thing doesn't show up, what will a small ding or rub do? the shorter the focal length and the higher the f-stop, the more there will be a change that you see it, and the danger of flare might increase, but if you use a hood this is neglectable.
of course resale value is diminished, but if you buy second hand, you can get fantastic lenses with tiny marks for good prices.