The Swift Bow’s horde clearing speed (and ammo efficiency, taking into count the total ammo) can rival the Hagbane in my experience (and no, the enemies don’t need to be in a single line, just a bit clustered or with some care, even quite sparse), and it needs far less careful shot placement than either of the other bows. That leads to it being somewhat better for emergencies with (unarmored) specials, and for simple reflex shots. True, its boss damage is nothing special, and its use against armor is usable at best. Unless you headshot, the Longbow is nothing special against bosses either, and similarly, Hagbane is not really better against armor.
On Handmaiden and Shade you’re very likely better off using something a bit more specialized, but on Waystalker, Swift Bow is very much a usable option. It’s the only one of Kerillian’s bows I can realistically have out for most of the map without being more of a burden than help to my team. I can use it (at least moderately well) against anything except Chaos Warriors, and those are the bane of ranged weapons anyway. So while either of the other two (or three, on Shade) are often more useful, the Swift bow is the generalist option.
In any game, having fun is in fact a quite strong argument. Not everyone wants to fully optimize or play tactical rock-paper-scissors. After all, people find their fun from different things, and having fun is the primary point of entertainment, games included. If I didn’t find the game fun, I wouldn’t play it. And yes, spamming arrows like there’s no tomorrow is really quite fun.