"Seems to me a filter is nothing more than a flat slab of glass ..."

Or possibly not so flat. Or possibly the glass used is not quite as pure as it can be. Or possibly the build quality is not as good as others. Or possibly its coating, if it has one, is not as even as it could be. If filters were just flat slabs of glass, you wouldn't have 3-4 companies standing out amongst the crowd.

Quantaray is the name used by Ritz(I think) to sell a re-branded budget line of lenses, filters and other accessories. When I first bought my Sigma lens for my film camera body about 4 years ago I also went ahead and got a Quantaray UV filter to save a few bucks (up until then, everything I had was Canon). The thing wasn't coated correctly, had too much of a red hue and flared at the first hint of cross light. Final straw for me was when the glue failed and the filter jiggled and rotated in the ring. That was just my experience. Not saying it's indicative of anything. Just that like with most things in life, you get what you pay for.

That said, do what you feel comfortable doing. It's your gear. I don't think anyone here really cares what you put on the lens (I have yet to have someone run up and look at the brand of filter on my lens). As long as you're happy, what else matters?

-=>Donald