The blizzard logic for rank 4 essences.

[locketto]

Am I wrong?

Are you a tank and you never be on a +15 as tank or healer? Don't worry have your "hardest to obtain" (pretty easy no) essence rank 4 for you, not skilled spec for free.
Are you a tank and farm rep, well farm 10000 quests to obtain your essences rank 4 for your off roles.

If you complete a +15 every week for an "X" amount of time you can have rank 4 essences for the main role and off roles for free or golds (tank/healer/dps). (free reward for no skill on off roles) (for example)

If you have a rank 4 for your main role from paragon chests, no rewards for no skill off roles spec and you need to farm it 3 times.

What is logic?

Why some rank4 essences for off roles can be bought and the others must be farmed?
Especially the ones which required skill to obtain them with one role can be bought and the others farmable no.