I'm biased, of course, because I often end up writing novelettes/novellas.
But I'm quite charmed by the way the romance genre, in particular, churns out anthologies of these mid-length works. Some are glimpses at a world expanded in other novels (which can be frustrating for me to track down in either direction). Others are simply satisfying tales in their own — as much as I love becoming absorbed into a novel, there are times I don't want to invest that much time into a story. And getting a mix of authors is interesting. There's a big-name author highlighted, and then three or so others thrown in, which is good exposure, I suppose...except that I often read the anthology for the sake of one of these lesser-known authors, and rarely consider the top-billed author's story the best. (Exception: "Alpha and Omega" by Patricia Briggs, in On the Prowl. I'm not sure I would have thought so had I not read Cry Wolf first)
Category romances are interesting because they're of a length that no other genre publisher would consider, I think. SFWA states that novels begin at 60,000 words, but the feeling I've generally gotten is that sf/f publishers would laugh at anything under 80,000.
Meanwhile, in the world of e-publishing, you have ebooks which can be of novella length because, I imagine, the overhead of publishing one isn't much different from that for a novel. I'd love to get my hands on some sales figures, though, and a graph of the price points for word count. Some submission guidelines have implied that longer works are preferred, and I could easily see readers balking at paying for something shorter than a full dose of romance.