Except Star Destroyers in Star Wars fulfill the destroyer role, unlike destroyers in Freespace. Think about what we see them doing: Chasing down freighters and corvettes during naval patrol duties (a destroyer's job) and escorting battleships (also a destroyer's job). We don't see them in line engagements except when they're forced into them, and
That argument might hold water except that it's not an Imperial Battleship it's a "Super Star Destroyer". So even if the ISD fills the Destroyer's role, why then is the Executor and others of her class also referred to as Destroyers? Does the Star Destroyer perform that role because that's what she's made to do? Or does she perform it because nothing else will. Expanded Universe aside, the Star Destroyer and the Super Star Destroyer are the only two ships we see in the fleet. Minor variations aside.
It should be noted, that in EpIV Han Solo originally calls the two destroyers over Tatooine roughly "Imperial Cruisers" (or maybe there was just one, but, same idea). As far as I know "Star Destroyer" only came into being for EpV. Or maybe Han said Star Cruisers . . . but same idea, Cruiser not Destroyer.
Another thing to remember is that the term "Destroyer" as applied to an ISD is simply a name
, not a class designation. The original designation was Imperator Cruiser
, which morphed into the most commonly accepted Imperator
-class Star Destoyer. Fans, over the years, have generally applied the class name "Star Destroyer" to any triangular shaped warship, such as the Victory Star Destroyer. Even then, Star Wars is a really bad example to compare to. It doesn't follow any "normal" class designations anyway. Ship class designations in Star Wars tend to be at the whim of the manufacturers.