Hi Wolf,
From my experience from 500px, a B&W category is very useful when you don't know where to send your picture... Often I wonder if my picture is a "street photograph" a "mood photograph"... not to speak about "everyday" which I still haven't figured what it really means!
That being said, I'm glad 1x doesn't have a b&w category because it doesn't really make sense... B&W is not a category or a genre, it's just a technical choice. If we had a B&W category we should also have an HDR, a X-processed and why not a lensbaby, a holga, and then also formats: a square, a panorama... etc. It'd be a mess!
I think it's more important to filter by genres like portrait, landscape... etc rather than technical choices like coulour, format or other processing choices.
My 2cents.
Have a nice week-end,
paco.