Everyone has their own view point on filters, and this filter vs. no filter discussion has been going on since I started photography, way back in the 1960s.
Back in the film days, it was to cut the UV blue, and I used to shoot at the sea shore (salt spray) a lot.
Today, I use a UV filter to keep the crud off my lens.
Just like the glasses I wear, it is sometimes amazing how much crud slowly collects on the filter. I feel more comfortable cleaning the filter than the front element. And if it is something that etches the surface/leaves a mark or is too hard to clean off, I can replace the filter with a new one.
And as has been mentioned, babies sometimes get their greasy/dirty fingers onto the filter, and that has to be cleaned off, in less than ideal cleaning conditions. I really try to avoid getting close to small kids, but sometime they come up running up to me and paw at the camera.
At my local high school, I use the filter as a disposable front element.
- The filter along with the hood is to protect the front element from greasy fingers. Every week, I used to clean fingerprints off the filters. Some of those fingerprints were really hard to clean off. I never understood how the kids got so much fingerprints onto the filter. Many of them did not seem to understand the message "keep your fingers OFF the front of the lens."
- And no matter how many times I tell them not to do it, they will use their questionably clean tee-shirt to wipe the filter.
- Front lens caps are constantly lost, so the lens are carried and stored without a cap, collecting grime and fingerprints.
- So, the filter is a last line of defense from the kids.
I discovered that some multi-coated filters are hard to clean off finger prints. Those were a pain to clean each week.
Now there are different types of multi-coated filters, some supposedly easier to clean.