Firstly, this limit of three herbs in a location was discussed but never implemented. And with the new evoke herb, it will not be implemented. I think that is a case of certain people shooting their mouths off about something I was considering.
Regarding the need for essences to evoke herbs, it is to encourage people to have gardens where they might grow herbs and poisons rather than just creating willy-nilly. If it's any consolation, Fiorella, I would not have put in this change had it not been for certain Sorcerers using their equivalent (EVOKE POISON) to create fifty jeggas and handing them out for free to their allies.