They're redundant. It's just a differently shaped dumbbell. Not necessary at all, if dumbbells are available. If dumbbells aren't available, kettlebells are a poor substitute for dumbbells, whereas if kettlebells aren't available and dumbbells are, dumbbells are an OK substitute for kettlebells.
"But what about kettlebell swings?"
NO. Just no.
Kettlebells are focused on compound exercises. I don't know where you came up with this:
If dumbbells aren't available, kettlebells are a poor substitute for dumbbells, whereas if kettlebells aren't available and dumbbells are, dumbbells are an OK substitute for kettlebells.
I can't think of any dumbbell exercises that couldn't use a kettlebell if it's all you had, but dumbbells are not well suited at all to kettlebell exercises.
And I don't know why you hate swings, they're a fantastic way to build squat form.