Why are Ryude's swords "so good"?

It's the same boost whether you're capping or not.
If you add one line to a one-line skill, you double your DPS.
If you add one line to a 5-line skill, you add 20% to your overall DPS.
If you add one line to a 20-line skill (or however many Dawn Warriors have), you add 5% to your overall DPS.

Dawn Warriors would do a max of 16 hits with Ryude's however max line cap seems to be 15 still so it would increase their lines by 1 instead of the normal 14 (For their mobbing skill not their bossing which seemingly isn't that good?).