Why is it that in many games, a player's 'ultimate attack' is gated behind some form of resource gauge? I'm not just talking about Final Fantasy as a series and its Limit Break/Trance/Overdrive/Quickening systems. You see it come up in other genres as well, including fighting games, and even team based PvP games like Overwatch. Why don't you just Omnislash on a fixed recast timer?
One thing that makes MCH and now RPR's resource systems interesting is that they have their burst windows set up as a sort of 'personal limit break'. The limitation of this approach is that you have to pretty much dedicate a resource bar (Heat/Shroud) to the one action. But that generally happens anyways. If you have two use-on-demand actions competing for the same resource, you'll just use the one that gives you more potency. If Living Shadow wasn't gated behind a timer, you'd never want to use Bloodspiller.
The other nice thing about resource systems is that they allow you to be a bit less predictable with your button presses, because little variations in gauge gains can result in you being at different resource levels on different pulls. This is especially true if you set up a number of possible conditions for resource gains, much like the limit break system does. For example, if DRK gained a small amount of blood every time their bubble shields absorb damage in addition to the standard combo based gains, you have a system that plays out a bit less predictably.
You can do the same thing with timers and resets, but it's a bit more heavy-handed.
What stands out about MCH and DRG's respective base combo systems is that they feel a bit different from the rest. After spending ARR juggling Path and Eye, most damage combo/maintenance combo sets feel very samey. DRG is different in that the combo chains themselves are quite a bit longer, and MCH's setup with Drill being roughly on a seven GCD multiple makes it functionally a variable four step combo of sorts. The common theme is that the order of the button presses gets swapped up in varying ways.
Either way, I think that we need at least a few jobs out there that aren't completely predictable in their button presses and force you to react dynamically to either what you have active (resets) or your current resource level. But players have to buy into the idea that a bit of randomness and unpredictability is good for us. It means that there's a higher mechanical skill cap.