Others argue that the front lens element typically is strong, and does not need extra protection. Some would even say that adding a filter may be negative, that it breaks more easily, and that the glass shards could add more to the damage of the front lens element.
Another argument is that an extra glass filter deteriorates the optical properties of the lens.
Now, I wouldn't want to drop my lens on the pavement to examine if using a filter helps protect it or not. Besides, as all lens accidents are different, one drop would not prove much anyway.
But to look at the resulting images when using a filter, as opposed to not using a filter, is possible. To examine the effect of using a clear glass protective filter, I took the same picture with and without a filter.
Example @ 200mm
Here is an example picture taken using the Panasonic GH2 with the Lumix G 45-200mm f/4-5.6 lens at 200mm f/5.6.
This is an image with fairly low contrast, and on first glance, the two images are pretty similar. Let's look at an enlargement:
So is there any difference? Not really, I would say. Perhaps one could say that there is a tad bit more contrast in the left image, without the filter.
Example @ 14mm
Another example taken using the Panasonic GH2 with the Lumix G 14mm f/2.5 lens at f/5.6.
In this case, we barely need to look at an enlargement to see the differences. Clearly, there is more flare when using a filter. The right image also has some odd "phantom lights" in the frame, from light reflections between the filter and the lens. Here is one example in the centre of the frame:
Beyond the flare, it's hard to argue that the right image is worse in terms of sharpness, for example.
Example @ 45mm
Finally an example picture taken using the Panasonic GH2 with the Lumix G 45-200mm f/4-5.6 lens at 45mm f/4.
In this case, it is very clear that the presence of the filter deteriorates the image. Not only is there much more flare, there is also a severe reduction in the contrast and sharpness across the whole frame.
Here is an enlargement which demonstrates this:
In low contrast situations, it is hard to find much evidence of negative impact from using a filter. However, when there is more contrast, e.g., a strong light source in front of the camera, the presence of a filter generates significantly more flare, and potentially also less contrast.
People who are proponents of using clear protective filters may still not agree with me. They could argue that if I had used higher quality multi coated filters, I would not have gotten these negative impacts. And perhaps they are right.
Since I don't believe in using filters for protection myself, I haven't invested in the most expensive filters. So it could be that higher quality filters would have reduced the deterioration.
Still, I believe it is fair to say that adding a filter will generate some deterioration of the image quality. But if using a protective filter gives you more peace of mind when using your expensive lens, perhaps that is an acceptable cost to you.
Personally, I prefer using hoods for basic protection of the front lens element.
The filters used were: "Kenko Digital Filter UV" (52mm) for the Lumix G 45-200mm f/4-5.6 lens, and "Green Digital Filter UV" (46mm) for the Lumix G 14mm f/2.5 lens. Both filters are littered with terms like "High Quality" and "Premium Quality", but I would take that with a grain of salt.