Your statement that a scratched and dusty printing filter causes no degradation of the image is not within the realm of physical possibility. Of course, if your standards are low enough you can print through a bottle bottom and you'll notice no degradation.
Thanks, E. You saved me a bunch of typing.
To expound a bit... I'll absolutely believe NB23's assertion that he saw no degradation, for the reasons you mentioned.
“Wise men talk because they have something to say; fools, because they have to say something.” - Plato
The OP is about the relative expense/quality merits of UV filters attached to the front of a camera lens.
It bares no relationship to plastic, dusty scratched Ilford filters attached to an enlarger head, which is probably the subject of a separate thread.
I am wanted a discussion about filters on the front of my camera lenses.