The use of 'protective' filters – either UV-blocking or just clear glass – are one of those eternal internet arguments that people do instead of looking at photos. Naturally, I have a point of view about it: I don't see them having any actual use. I'm not about to say that anyone who disagrees with me is wrong – this is just my reasoning, and of course everyone else should ignore me and do whatever they're comfortable and happy with.
• Filters are the cheapest element in a lens. They're a high-margin item, so it's guaranteed that every step in the chain is making a healthy profit on it, and the materials and fabrication costs are going to be very low relative to the lens itself.
• By necessity, filters are the largest diameter glass in the lens, and are held in place only by narrow metal edges.
• In addition to their large span, filters are flat glass, rather than the domed shapes of the lens elements.
All of this adds up to a filter being the most fragile part of a lens. So to say "I dropped my lens, and the filter saved it" is a false syllogism. Just because the filter breaks on impact doesn't mean that the lens would have if it was absent. And just because all of the glass is intact, even if it's including the filter, doesn't mean that the impact didn't damage the internal mechanisms.
An analogy: Cars have a lot of safety systems built into them. Saying that a roll cage, airbag, or seatbelt saved a life could be a perfectly literal statement. Saying it about the bumper is not at all the same thing.
The other "protection" that a filter is supposed to provide is against scratches. I have talked to one person who had a small fleck of the surface coating flake off of a lens, but scratches and damage are exceptionally rare. Modern glass and coatings are very, very tough – including, incidentally, the ones on cheap filters. I once got my hands on a really cheap one – it was visibly off-colour and had a warped filter ring, brand new – and I tried to scratch it by hitting it with my keys. No luck. I eventually took my pocket knife to it, and still couldn't mark the front of it. If a no-name filter that couldn't have cost more than a dollar to produce can withstand that, then I don't see much reason to worry about the front element of a lens from a reputable manufacturer.
As a further example, I wear eyeglasses every moment that I'm awake, and have for twenty-four years. They've always been anti-reflective coated plastic, far softer than the glass in camera lenses. They're exposed to all kinds of abuse and neglect, worn in all weather, and I'll clean them more times in a day than I'll clean a camera lens in a month. Out of the half-dozen pairs of glasses that I've used in that time I can only think of one that I've scratched.
I do keep a hood on all of my lenses, which can provide both genuine impact protection and image quality benefits, but I rarely use lens caps. Two dozen lenses (or more) later, I've never seen a scratch or a fleck – and that includes on my used lenses, some of which have been older than I am.
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