One of the (imo) foremost advantages of controller as a service, is the fail fast principle. This means that if you're missing service definitions or try to inject something that does not exist, it will throw an error during compilation. When using the (Base)Controller provided by Symfony, you won't find out if the service exist until you call get() run-time, which might happen once in a blue moon. It also makes it impossible to use private services in 4.0.

http://symfony.com/doc/current/cookbook/controller/service.html