Stacks aren't really a resource that you "manage" per se. Everything costs exactly five stacks. Your resource management ability largely hinges on your ability to count to five. You then either perform your high damage single target move, or your high damage AoE move. You can learn to play WAR HUDless if you know how to count in your head (this is pretty good practice, in fact).

Part of the problem is that the Deliverance stack moves introduced in HW are significantly better than the Defiance stack moves. In ARR, Inner Beast forced you to think about whether to spend your stacks immediately for offense, or to hold them for mitigation a few gcds later. With the introduction of Fell Cleave, you don't really want to spend your stacks on Inner Beast if you can avoid it, and you have so many other defensive cooldowns with such short recasts that you generally can find ways around using it. Outside of progression, it's pretty much just a safety net for when you mismanage your defensive cooldowns. I would be surprised if Steel Cyclone gets used any more with Decimate available. Unchained is interesting, especially if you were actually forced to use Defiance for a section of a fight, but more often it tends to be relegated the role of enmity grab.

Most of WAR's gameplay hinges around Berserk windows every 90 seconds, much like PLD hinges around FoF windows every 90 seconds. The entirety of both jobs depends on how well you place and execute these relatively fixed rotations.

If you want to give WAR a more interesting resource management system, you need to introduce variable costs. Introduce some abilities that cost 1-2 stacks. Bring back stack penalties for changing stances. Don't give away stacks so freely. Promote defensive stack usage, either by making IB required, or adding stack costs to some other defensive abilities. Perhaps make the passive effects of the stacks more desirable, forcing you to stay above a certain threshold. Force the player to think carefully about whether they want to spend their stacks now aggressively, or whether to hold them defensively for a necessary ability in a few gcds.