Is Dull Ranged worth it?

If it was a different type of skill, it could be better, but since it's a B skill usually there are better options. If you're using +spd and Raudrblade you use Desperation. If you want a debuff build with Grima's Truth you use Wind/Watersweep. You can also use Red Tomebreaker/Sworbreaker or even Quick Riposte if for some reason you have a -spd +def(for example) Morgan and want to make him a bulky Raudrowl user.