Controllers need to provide extensibility, much like Model, but can't use the same mechanism as the pre-requisites and database with loaded modules.
Controllers are provide their own extension mechanism, separate from that of models:
Controllers are created by inheriting from

class openerp.http.Controller

and defining methods decorated with route(): For example code is below.

class MyController(openerp.http.Controller):
    @route('/some_url', auth='public')
    def handler(self):
        return stuff()
And override a controller, inherit from its class and override relevant methods,
class Extension(MyController):
    @route()
    def handler(self):
        do_before()
        return super(Extension, self).handler()

Note- Controllers thus provide their own extension mechanism, separate from that of models.