When my dad was getting a new TV, he went to specialized A/V stores to compare them side by side, playing the same videos to check the little difference they have. Sure, if you put them side by side, you notice the difference, but when else do you actually get to compare performance like that? Does it really matter?
So can you tell if a photo is with or without a UV filter?
Do you think about whether a photo was taken with or without a UV filter when you look at it?
If the answer to both is "no", then why should this even be a debate in the first place?
I use UV filters on all of my lens and I don't think about photo quality like that. However that's just me, and you have your own ways to make you happy.
What made me happy was when I took a photo of the rough sea the other day, I was able to capture the shot I wanted without worrying about "protection". All I had to do was to clean the salt spray from my filter that I forgot was already on. Hood? Sure it reduces lens flare and improves contrast, but only useful under specific conditions, having it on all the time limits me as a photographer. To say that hood is good as lens bumpers is a rather hard one to sell for me. One should be aware of his surroundings to begin with, and unless someone else hit your lens, all damages are due to YOU as a careless user. That goes the same for the guy who's friend dropped a camera on a crappy tripod and ended up blaming the filter. Seriously?
When you get a good photograph, you want the photographer to be credited. However you blame your equipment for bad quality? Come on!
To me, a filter and a hood are entirely different equipment. You cannot see one as a replacement as the other. It's apple and orange. No filter user really use filter as a "bumper". I certainly don't think I can be any less careful when I use a filter. To me, a protective filter allows me to do more, not less.
I've taken photos under situations when I need both. I think when you compare them, you are already taking them out of their intended context. If what you really care is about bump protection, well, wrap your lens with bubble wrap or something instead.
In the mean time, I'm going to keep on posting photos taken with UV filter on. May haters continue to hate!