Hands up those that keep filters on the front of their lenses for protection.
Now, those with your hands in the air (it's ok, you can put them down now) do you use the best filters you can buy? You know, the Canon range or the Hoya Pro 1 Digital etc?
No? You use the ordinary ones from the corner shop?
Shame on you. Let me put it to you simply. A filter creates another air-glass interface and increases the risk of light scatter, reduced light transmission, flare, ghosting, aberrations etc. If you don't use the best filters you can buy, you may as well give up on the expensive glass habit you have and buy cheap milkbottle lenses for all the good you're getting out of your L-Series ones.
More to the point though, what protection is a filter on the front of the lens going to give? Honestly, if you drop it, two things will happen. The filter will break, thereby scratching the front element anyway, and the filter mount will deform leaving you with a broken filter jammed to the front of your lens. Great scenario!
In my opinion, there are only a couple of occasions when using a filter on the front of the lens is acceptable. 1. On a beach or dune with wind-blown sand. 2. In a rain storm with driving rain. 3. When the lens is pointing out of a fast moving vehicle in the direction of travel.
Notice anything about these scenarios? They all involve the possibility of wind-borne objects hitting the front lens element. At all other times, your better off without.
If you really want to protect the front lens element, get a lens hood. Lens hoods should be bought with lenses as standard. In fact, we wish Canon would start bundling them with all the lenses, not just the L-Series.
The lens hood, while not as effective against wind-borne objects, is effective against dropping the lens (it acts like a crumple zone), fingers getting the front of the lens accidentally, and most importantly, lens flare. Using a lens hood should be a matter of course. It will improve contrast in your images, reduce flare and ghosting and give you that piece of mind that for most situations, the front of your lens is well protected.