Spaceship and Sword
These two tropes combine, above all others, to signify space opera - the real thing; statuesque, full throated, with a bronze bra and a spear. At SFConsim-l we may look down Sir Isaac Newton's nose at "operatic drives" that don't require a ship to fly backwards in order to slow down, but plenty of SF implies such drives without being truly operatic. Once we see the flash of blades drawn, however (and they are too long to be "knives") we know we are in the operaverse.
As with space fighters, Star Wars greatly boosted the popularity of combining spaceships and swords. The theme is so much older, however, that Clarke had some fun with it back in the rocketpunk era, in Tales from the White Hart. (The story, as I recall, is "Arms Race," and specifically involves Hollywood.)
Ultimately the combination says everything about why SF and fantasy are in the same part of the bookstore. It isn't just that both are subgenres of Romance - so are mysteries, historical fiction, and romance in the usual sense, none of which are grouped either with SF/F or each other. Science fiction and fantasy are more nearly two facets of the same subgenre: tales of evening isles fantastical.
Yet close kindred though they are, spaceships and swords seem to belong to evening isles so far removed from one another that the very keel of the universe, the willing suspension of disbelief, is strained to its limits by trying to combine them. Is it possible to do it at all, now, without doing it tongue in cheek? (Comments to this blog touched on the subject here.)
Many years ago, Poul Anderson found one solution in The High Crusade - he thrust a group of medieval knights aboard an alien spaceship, more or less, and left them to cope as best they could - rather like SCA members stumbling into a Trek convention when they had no idea that Trek existed. Since these particular medieval people were nothing stupid, with a lucky break or two they coped pretty well. This is a valid solution but a pretty specialized one, and I'm not sure but that Anderson hasn't already covered the waterfront here.
I hint at an almost-solution above, in my mention of blade length. Knives are multi-purpose tools, and one of those purposes is close-quarters fighting. (A related one is shanking someone without warning.) It seems to me that spacehands might frequently or even routinely have utility knives on hand. To these we can add fire axes, and other such working tools as cargo nets, that could come in handy for other than their designed purpose.
I'm not quite sure quite how plausible space station barroom brawls are, or the ever-popular waylaying down in the service corridors, but I'm not quite ready to say that these things aren't plausible. People have a fairly impressive capacity for getting in trouble.
A sword, however, is not just an extra-long knife: Worlds of connotation separate swordfights from knife fights. Are there plausible circumstances, other than a High Crusade variation, that can justify swords aboard spaceships?