Arms does have filler though in the way of Slam (or Whirlwind if you take Fervor of Battle), which affects up-time on Trauma or Rend. The problem is that rage generation currently doesn't allow for much use of these abilities. As I've said before, this could all be just because of the shitty starter gear they've stuck us with, but seeing how bad it feels with only 10% haste makes me think it's going to need another 20% to feel good. Being that dependent on a single stat is just going to create the same scaling issues they've been trying to avoid. Ironically, it has usually been Fury with these scaling issues but they've mostly addressed them with the recent BfA changes. Now Arms is saddled with them. They need to go back to the drawing board with Rage as a mechanic, as well as redefine the Arms class fantasy. "Slower but heavier hits" doesn't mean shit when you have to stack a ton of haste to negate the "slower" portion in order to perform adequately. I would rather Arms have a more complex rotation than Fury's wild swinging for the fences. Arms should be the "smarter" spec compared to Fury's button mashing, like the difference between Frost and Unholy DK's these days.
Either way both Warrior specs have filler, or weaker buttons we push when we don't have stronger buttons to push. I think we need these weaker buttons for the sake of rotational depth and balance, but obviously we want to limit how often we have to push these weaker buttons. As you said, with the way Fury has been playing, Furious Slash hasn't seen heavy use in recent days but I like the way that Frenzy works with the weaving of FS to maintain its buff. I think that is a great active talent for adding some optional complexity to the rotation. Arms gets to choose which filler it's going to use between Slam and Whirlwind which affects what bleed it's going to use, but none of these choices add any additional complexity apart from Rend, which has a minor impact at best. All of these abilities are tied to rage generation so there's not really much active thinking going on with Arms. It's all just passive and your rotational choices depend on Tactician procs and what abilities you can press based on how much rage you passively generated, the latter of which has proved to be problematic for the entire class in the past, but it seems like they've only figured it out for Prot and Fury while Arms seems to exist in a vacuum. But I digress....