Allow me to throw a spanner in the works...
Concept 1 - Degrade as least as possible
Assuming that you're not using a premium lens, a lot of compromises would have been made, both optically and mechanically in the lens. Therefore, it needs all the help it can get, and using the very best quality filters will help in any way possible (assuming that user wants to always fit a filter on).
Concept 2 - The lens is not a top performer anyway, why waste money?
Also holds water, because you can only do so much to help a less than ideal or lower quality lens.
Concept 3 - Justifiable depends on a particular lens, and shooting conditions
Purchase of ultra-high quality filters depend largely on shooting conditions, e.g., how often do you shoot into light sources, towards the sun etc? Will you see the difference between a $30 Hoya and a $70 B&W?
Concept 4 - All filters degrade the image, so it's better not to have one
I tend to notice that I take off any filters unless they are absolutely necessary - either to prevent possible damage to the front element, or when the use of a particular filter is essential to getting a picture.
Concept 5 - If I'm spending that much on a filter, I'd rather get a manufacturer's filter
Originally, people only bought B&W as an alternative to buying a Nikon or Canon filter because B&W were still cheaper, but now that the prices are so close, it doesn't make sense to get a B&W when I can get an original which has the (supposedly) same optical quality glass and coatings as my own lens. Also bear in mind that there was a lot of talk about different grades of B&W filters in the open market so how do we know for sure we're only getting the top quality stuff?
Aside - I've found that oil and finger-print smudges are just as easy to clean on a Nikon and Canon filter as a B&W. Pentax SMC filters were more problematic - like Hoya.