Why All the One-Button Mechanics?

[Shurrikhan]

Mana Management - Eclipsed in most content and in most cases by hitting on CD a button that typically serves no other purpose.

Knockbacks - Mostly eclipsed by hitting a button that, for all but one role, serves no other purpose.

Positionals - Eclipsed by hitting a button that serves no other purpose.

Etc, etc.

...If the mechanics are worth having, why do we then bloat our bars just to either (A) ensure they're typically non-mechanics or (B) bypass any need for polish to said mechanics?

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Consider it this way:

Is MP a worthwhile mechanic? Is the game better off for having it?

If so, is the game also better off for having devalued all parts of managing one's MP besides hitting a certain button once per minute?

If not, why is the button there?

Similarly, are knockbacks fun to have in the game? Is the game better off for having them?

If so, is the game also better off for most knockbacks being a matter of hitting a certain button, rather than pre-positioning?

If not, why is the button there?

We're increasingly told that previously held and often more meaningful skills, mechanics, etc., need to be pruned due to limited bar space, but at the same time, we're spending bar space specifically on buttons that do nothing but remove mechanics we've already spent time developing or devalue related play that we seem to generally think the game is better having than not having. ...Why?
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:: I am not advocating for the removal of these MP, positionals, displacement, or the like.
:: I am not asking for even more buttons by which to make mechanics into non-mechanics.
:: I am advocating that we not spend valuable button-space on watering those mechanics down and would assert that if those mechanics are worth having at all (and I believe they are), then they're worth polishing and making wholly meaningful.