plone.directives.form is a product of effort behind Dexterity, so it also shares same philosophy.:
Leave simple things simple and make impossible things possible.
Well I’m not sure if I made this up or I heard it, but if it’s not like this it should be.
Convention over configuration they say. And they are right. Almost I would say. In most usecases where you build / extend code you normally want to write as little as code as possible and get the results quickly.
Then other time you are writing app / products / package / framework which expects to be extended in some parts. For that I expose configuration in ZCML.
Thumb rule I follow:
Everything you want to be extendible put in configure.zcml
While grokking might save you some time, it might give you big problems when you will face the task of debugging it. But don’t be scared ... its all Python.