Long-time readers of this blog know I write up recommendations of a few short stories each month I really enjoyed. "Sic Semper, Sic Semper, Sic Semper by Carl Wiens" was my favorite story of the year. The first line of this story pretty much sums it up: "The time traveler set up a studio apartment in Abraham Lincoln’s skull in the frozen moment before Booth’s bullet burst through and rewired history," but I also enjoyed "The Girl Who Escaped from Hell" By Rahul Kanakia and "Our Talons Can Crush Galaxies," by Brooke Bolander. As a participant in Arisia' "Short Sharp Shocks" panel, there are the stories this panel might have been written to address.
I got started reading short fiction through Stephen King and Isaac Asimov. That's probably why I have the assumption that short stories and longer works are both equally part of a writer's medium, that neither expression is more important that the other.
I recommend reading short stories to everyone. I actually have trouble figuring out why short stories are not more popular. It would seem to be a natural fit for the modern pace. A good short story has the power to conjure an entire dynamic, compelling world into being in 5,000 words or less. Particularly in the case of speculative fiction, I'd argue that gives the writer the power to dramatically up the stakes and flexibility of a story. A character or situation that might wear out an entire novel can comfortably fill a smaller piece.
Having grown up on Clarke, Asimov, and King, I'd say classic short fiction has a lot to say to modern fans of speculative literature. It doesn't take much digging to realize that a lot of the classic science fiction, fantasy, and horror ideas got their start in short form stories. "Nightfall," "The Sentinel," and "The Lottery" all feel like complete statements, pushing the tradition of speculative fiction forward every bit as much as their writer's more celebrated novels.
So what it that makes short stories worth the effort to read? Their length is both their defining characteristic and their essential strength. As I've suggested above, a short story can fully explore certain ideas that might struggle as novels: too little concept over too many pages. Also, there is an intensity to the stories of short fiction. Novels can build up to impressive spectacles and spend considerable time on the slow build of tension. Poetry can capture perfectly a single moment, emotion, or impression. But the purpose of short story is to provide a single unforgettable experience. Because a short story can usually be read in a single sitting and considered in whole, a great short story doesn't just entertain, it has the sense of occupying fully a human brain, as though for 5,000 words or so the only considerations possible are those wrapped up in this one brief story. For an example of what I mean, I'd earnestly urge you to find "Exhalation" by Ted Chiang. There are plenty of short stories that could change a person's life. Exhalation is one of them.
In no particular order I'd also suggest the following authors as showing the possibilities of speculative short fiction in the 21st century: Laird Barron, John Langan, Ken Schneyer, Ken Liu, Gwendolyn Kiste, Maria Haskins, A.C. Wise, and Kristi DeMeester, among very many others.