[eluser]BrianDHall[/eluser]
$this->News->add(); and $this->News->update();, as what is being added or updated often makes a big difference on how you have to handle things. If you end up with News, Comments, Users, Articles, Messages, and more - that would be one hell of a big Main->add() or Add() class. And as adding and updating are often closely related, it can cause a lot of code replication to have an Add() class and an Update() class.
Much better to encapsulate the class/noun, as jedd suggests, and its related functions (methods/verbs). This meshes with MVC and OOP best practices, and doing it any other way will violate everyone else's expectations of how things should be coded, if you should ever want someone else to help with your code or sell your application.