I generally see both flashes and sgian dubh as decorative, in that they are not really nessesary to the outfit, but are certainly traditional to it.

That being said, I wear both regularly. Even though I'm pretty sure all my kilt hose will stay up on their own, I wear flashes as a way to add color, and complete the traditional look.
Same way with the sgian dubh, If I'm wearing hose and flashes I wear a sgian dubh too. But even though mine all have nice enough blades, I tend not think of them as useful knives, and usually carry my everyday folder or Swiss Army Knife in the sporran. There are no legal issues where I'm at, but your milage may vary.