Then we would have aggro priestess.
Which is absolutely fine.
No archetype should be excluded from any type of play style / book construction.
My group came to the conclusion about the meditation amulet. It is not about the mana it produces but the option to produce it.
From the wearer's perspective you can more easily react to an opponent's play when you have the ability to generate 3 (unpreventable) mana now. It adds stress to mana-lock decks. Will it stop a solid mana lock? No but it does help with a nudge here and a nudge there.
When you are sitting across from one it is much harder to read an opponents play. When they have a battle forge, sectarus, libro, graveyard, and pentagram out (an exaggerated theoretical situation ) with a myriad of mana across all that and 5 extra prepared spells on the board I can not realistically anticipate every play. That is compounded with the meditation amulet.
It has some benefit in that mind game aspect of the game. It is much harder to try to plan what your opponent is planning when they have an extra 3 mana they can produce at will.
If your opponent tightens their mana budget to anticipate a card you never actually use, than I would argue meditation amulet has paid for itself and then some.
All that to say
I understand it's purpose, and the reason they wanted to include it, however I do not think that this card is going to make a lot of noise right now and I am not a huge fan of it. But who knows, maybe in 3-4 sets this card will be the lynch pin of every book?