The naked or not debate continues! I think maybe a filter is a tool for a time and a place? I bought into and so far follow the 'protection' thought line. I do agree if you are going to cover use quality I use Hoya smc and am happy so far, I do have a couple of Tiffens. I would think that with a CPL that quality would be much more important then filter quality? At this weeks ball games I am going to use the Tamron 70-200 unfiltered and see how that looks, and if it has any effect on some of my 'lack of focus' issues LOL
Yup, if you are going to use a filter, use a good one, you want it to work(transmit light)don't you? Tiffen has a reputation for making some of the worst filters out there, basically utter junk, poor in contrast, and terrible in flare. If you think you need a filter buy a good one, you've just spent a large amount of money on that lens for good optics, and now you're going to put a coke bottle bottom in front of it? Funny. Great optics deserve to keep their greatness, and putting a $30 window pane in front of them is not doing the lens' optics any justice.
The idea of the filter for protection was invented by the photo shops as a quick POS(point of sale)item with a high profit margin. While you are still in your sticker shock haze at what you just paid for that lens, a salesperson says "you'll need a filter to protect that new lens investment, won't you?" Yes they can help protect your lens(from dog nose prints, finger prints, salt spray, blowing dust/sand/ect.), but from a drop they offer little or no protection. They are excellent at doing what they were designed to do: filtering light, provided a decent MRC filter is selected.
The front element is far far stronger than any filter is, the filter will break at the slightest impact, while a front element can take quite a whack, and with a hood on, a front element would be pretty hard to smack. I personally have seen front elements with huge hacks in them and the lens still performing very very well. I've owned one, an 80-200 f/2.8L with a half inch by 1/16" wide and deep scar in it. The photos from it were awesome, I couldn't even tell there was anything at all wrong with the front element from the output.
Here's an example of how much damage a front element can take and still take OK photos. Sure the photos are about the grade of a cheap disposable camera, but look at the damage to that front element! You wouldn't expect to be able to get any kind of photo out of that lens, yet the photos are passible, the text is readable, and as stated in the article most of the IQ loss is probably due to a shifted element from the impact. No filter would have helped one iota in that damage, except by adding another layer of broken glass to grind into the front element potentially adding more scratches to the already seriously damaged element.
This argument will go on forever, I have NEVER seen any evidence anywhere that a filter has saved a lens from damage. All I ever see is anecdotal evidence in threads that are always started by folks justifying their filter for protection stance. The so called evidence is always "my filter is smashed and my lens survived therefore my filter saved my lens". There is never any shred of proof that if the filter wasn't there the lens would have been ruined. I've never have seen a thread started by a naked guy talking about how his hood saved his lens, those posts are always in response to filter for protection threads. The hood just works, even for a flying object to get in there, it has to be aimed pretty well. In a fall the hood takes the impact and spreads it back to the lens body. The front element isn't even involved, with a filter, the front element will very likely be involved, whether by collateral damage, or direct impact after it breaks with very little force.
Yet in the opposite vein, I have seen countless thread in which someone is complaining how their expensive XX-XX f/2.8 lens is performing poorly, flaring, back-focusing, front focusing, hazy, ghosting, etc. The question is then asked, "do you have a filter on it?", the answer is almost always "yes". Then the poster is asked to try it with their filter off, and almost always the trouble goes away.
If you feel you need a filter for whatever the reason you think you need it for, get a good multi-coated one. Hoya Pro-1 MRC, B+W MRC, ect. A cheap filter will never do that expensive lens justice, and in many cases will seriously degrade the wonderful optics the lens already possesses. Every filter changes the optics of your lens, however small, it still changes the light transmission of the lens it is placed on. In most cases if a good filter is used, that change is not at all noticeable, but it is measurable, and that's enough for me.