Disclaimer: I have not read through all the answers yet. But I do have a strong opinion on this subject, based I believe on experience and accurate measurements.
A UV filter always affects image quality, including sharpness. It lowers performance in sharpness, contrast, transmission, etc., by at least 5%. For some filters, more than 10%. That's doesn't sound bad considering that the Sony SLT cameras absorb 33% of light (transmission only), but here we are talking about a 5% or more reduction in every optical characteristic of the lens, not just transmission.
It gets worse the larger the area of the filter. So for 18-55mm lenses it's not really important. But for a 300mm f/2.8 lens, it would be ridiculous.
A 5% reduction across the board is enough to take away the $5000 benefit of owning exotic glass.
I never use UV filters (unless I am taking photos directly in the face of flying rocks or other objects--I do own filters for that purpose, and of course other filters that have actual purposes). There are clueless photographers who won't buy used lenses from me as a result, and they are the same people who set lenses on their side rolling all over the place because they are afraid to set them face down the way they were designed.
Also, I clean my lenses very sparingly. More damage is done by excessive cleaning than by any dust or dirt on the lens. I have had dirt cascade into the front hood of my 300mm and get on the lens, and it was barely noticeable in the images. Take note that the dirt doesn't reflect light very well, so the image quality degradation was only in proportion to the actual square inches of the grains of dirt that were on the lens. It was a much smaller effect than caused by a UV filter. Dirt has great anti-reflective characteristics, I have observed. So by putting a UV filter on your lens, you are basically doing worse than covering it with a light coating of dirt.
And my lens was fine. Just a brush off and it was indistinguishable from the day I purchased it.
