In Ruby, since you can include multiple mixins but only extend one class, it seems like mixins would be preferred over inheritance.

My question: if you're writing code which must be extended/included to be useful, why would you ever make it a class? Or put another way, why wouldn't you always make it a module?

I can only think of one reason why you'd want a class, and that is if you need to instantiate the class. In the case of ActiveRecord::Base, however, you never instantiate it directly. So shouldn't it have been a module instead?