Viability of a Spell Speed WHM set up?

[Donjo]

Except, y'know, having enough Piety to comfortably heal for many minutes of constant fighting, or a Scholar's unchallenged need for Crit rate...

Piety's not a Secondary. I would argue that it is a full on Primary Stat. I didn't factor it into that statement.

When I said "meaningless", I meant this for the reason I stated in a further sentence: if you had zero of every secondary, content wouldn't be any more difficult. Healers have such high potency skills that main stats alone get their heals well above what they need to be. The only thing Secondaries can be observed to do on a healer is help to save someone from death with pure, dumb luck. Any skilled healer stops needing them very quickly.

Spell Speed: You might save someone with a split second heal, but it's rare. It's more effective to pre-cast anyway.
Crit: You might save someone with a clutch critical, but it's rare. You should fight battles assuming that you will never crit in the first place because it's bad to rely on RNG. Thus, most crits end up being overheals anyway.
Det: You might save someone because that Cure healed for 50 more than usual, but it's rare.

All of these situations should be avoided entirely in the first place.

Now, Piety. I consider a stat to be "Primary" if it is evident that not having enough of it will cause you to be physically unable to complete content. Thus, I consider the following stats to be Primary, in whatever capacity the various jobs use them: Strength, Dexterity, Vitality, Intelligence, Mind, Piety, and Accuracy.