Documentation says:

The Grails team discourages the embedding of core application logic inside controllers, as it does not promote re-use and a clean separation of concerns.

I have one API controller and a few Groovy classes in src/groovy folder. Those classes just implements my application logic so actions in API controller works in this way:

//index page
def index = {
    render new IndexApi().index(params) as JSON
}

I'm curious - is there any reason to move my application logic from plain groovy classes into services ?