That depends entirely on the reward. Clearly even with a 30% bonus to oGCD damage and a relative ~47% increase to weaponskill damage over time during SB, it was still quite able to be utilized as a mechanic. Even now TK openers are viable.
Yeah, and they are a pitiful waste of a would-be mechanic and/or button. As buffs with no levels nor spending interactions, one has to wonder why their current version bothers to exist at all. But therein lies the question. Do you mercy-blow any and all of these mechanics because they've been so gutted, or do you revitalize them?
In combat, -- vs. ...what, out of combat? -- speed manipulation is hardly the issue. The issue is control. Getting an Arrow half-way through your rotation, when you were not ready for it and now have no way to utilize it, feels like poop because there's no way to utilize it even though it takes up part of your focus. It will consume mental resource at no gain. Having control over it, however, provides a very different story. For my own part, I loved it. It let me pull off even more would-be overextended rotations, allowing me to triple-Kick during PB before I had the SkS to do so by default, or allowing me to get in an extra Aeolian into TK window or skip Fire altogether on by BLM, each fun rotational differences. And with the number of BLMs who entered an AST party with "Hi. I'll be taking all your Arrows," that hardly seems unique to me.
While that might sound like a similar direction, it'd most likely nearly antithetical. There are only three ways such a system could go:That's why I'd prefer that stances remain more or less their own things, albeit far, far less niche (or dominant, in the case of Fire until GL4 and Wind thereafter) than they are now. They can have an impact on speed, sure, but making them only a stick shift for speeds will end up with a Cataclysm-era Warrior debacle (whereby every skill was just macro-bound to a stance-change) with unnecessary bloat.
- There are too few restrictions on stance changes to prevent someone from merely using high-damage-low-speed on each high damage-per-GCD skill and low-damage-high-speed on each skill with low damage-per-GCD (not including buff effects), forcing one to swap stances as often as possible to better juice Demolish and Bootshine.
- The stances are too restricted to have any useful additional effects, and likely thwart gameplay that, too any player, would seem an obvious use of the stances, making them feel unnecessarily bloated.
- Some miraculous hybrid of the two that manages to axe either side's issues -- possible, but horribly unlikely.
We can have just as much choice and flexibility in speed without needing that bloated approach to stances. Consider my oft-repeated suggestion of having GL work like HW-era BotD, where all relative potency builds GL duration (called "Ki") but only Coeurl skills move you up to the next GL level (with GL duration at or above 10 seconds providing the next level's bonus to the Coeurl skill that levels you up) and GL duration (Ki) can be spent to add additional effects to your damaging abilities. If you want to go the stance-specific route, let's say that Wind will "rush" the ability, making it usable before its cooldown is ready at cost based on its potency-per-second time number of seconds rushed, Fire empowering the ability, for a damage buff at variable consumption, and Earth "fortifying" it, causing its damage to grant rapidly fading damage absorption, which grants bonus potency upon consumption that does not stack with positional potency bonuses (thus effectively guaranteeing positionals and leading to seriously chunky Bootshine crits). In such a case, there's no need to stance-dance constantly just to milk particular skills; instead, it's all about the flow and sync, allowing you to align the perfect Internal Release, Perfect Balance, Riddle of Fire, or what have you.
Now bring back the suggestion that GL stacks should fall off one at a time. For context, let us say that we go back to the old 5% Damage and 5% Attack Speed per stack. When increasing GL level, the speed at which GL drains over time and per ability enhancement (the spending on which is completely optional in Wind if you go the stance-route, or at all times if you do not) likewise increases by 10%, compounding (for a 1.1, 1.22, 1.33x, or perhaps even a harsher 1.1, 1.3, 1.5x, consumption). It therefore become a risk-reward system: Do I want to remain in GL3 just for my auto-attacks, Twin Snakes, Dragon Kick, and Snap Punch, or would it be worth spending Ki now, dropping a level, and then maxing it again before my next Coeurl skill for full Bootshine, True Strike, and Demolish damage? Unlike TK, you don't only have a mathed-out single answer that utterly changes your playflow; you have granular control, all without even needing any additional keys.
In such a system, intentionally overspending on additional effects for your abilities could easily and smoothly down-level you, yet that could be used tactically rather than as a fail condition. If you would otherwise outpace your Twin/Dragon/Demolish buffs and have no abilities left in that rotational string to benefit from GL's damage buff, you could freely downlevel yourself in order to time those reapplications more closely, neither clipping nor delaying Demolish, etc, all while conserving Ki.
No extra buttons would be necessary just to manipulate your speed: you'd have constant, granular control -- just through your plethora of core abilities (including a returned SP, HF, and the whole gamut).
* In this, TK would be a raw Ki spender, worthwhile at the end of CDs, before going into downtime, or before PB. That said, padding enough Ki (GL duration) onto your highest stack could technically cause it to have no cost to GL stacks, either dipping down a level only just before a Coeurl skill with 10+ seconds duration (i.e. giving the benefit of the next benefit on that Coeurl skill as well) or leaving you with a sliver of duration left even on your highest stack. That wouldn't typically be optimal, but it'd be damn close.
I think those two things would be helpful indeed no matter what direction Monk takes.