Why do we check for the controller file?

[Duane Johnson]

I remember having this discussion on IRC before, so Ezra if you could  
remind me that would be appreciated:

Why do we check for the existence of the controller file in (what is  
now) Request#controller_class?

The reason I ask is that I would like to implement some sort of  
"Rails engine" like features for gem plugins--in other words, drop in  
a gem that has controllers/views etc. and have it just work.  In  
order to do this, I need the Request#controller_class to return my  
gem's controller class (which corresponding file does not exist  
inside the standard controller directory).

Thanks,
Duane Johnson
(canadaduane)