I'm a little confused by your post. How is saying “Be careful about starting to collect swizzle sticks… it’s a slippery slope,” an egregious use of the English language? If the person is arguing that a chip collector adding swizzle sticks to their collecting interests may go on to collect matchbooks, napkins, ice buckets, etc. it seems to me it is a proper use of the term. I agree it is a specious argument since everyone who collects swizzle sticks won't go on to collect many other things and perhaps end up with a house overrun with collectibles. But although the term is often used in questionable cases, I don't think it is a misuse of language so much as faulty logic.