The cost/benefit of learning multiple content management systems

I'd say that a thorough knowledge of one CMS is more important, you're just so much efficient if you really know what you're doing. Drupal is also very flexible, you can do just about everything with it, even though it sometimes is quite complicated.

I don't think you gain much by learning a similar CMS, but it could prove usefule to know a CMS that is very different from Drupal. For example a very simple CMS that you can use if you just don't need all the complexity Drupal brings in.

I'd say a bit of superficial knowledge of a wider range of CMSes won't hurt you. You don't have to become an expert right away, but knowing their particular strengths and weaknesses and the mindset they come with might prove handy. When you decide you need one of them, there's usually still enough time to dive into the documentation and make it work.

If you're a PHP developer, you might find your development skills can stagnate if you only code in Drupal all the time. You're not exposing yourself to different methods of software development. For example: Developing in Drupal 6 doesn't give you much experience in Object Orientated Programing. It's largely procedural (with the exception of Views).

While Drupal is a quality product, there's definitely a misconception that it is flexible enough for everything. I've been developing high profile sites in Drupal for some years and I've seen quite a few projects developed in Drupal that should have been done in another framework or language. Some of them have. I would diversify. You'll be able to make better decisions.

I'm currently branching into Ruby on Rails and I'm finding it's giving me some valuable perspective. It's also an excellent framework for those projects where a CMS may not be appropriate. Also: Some of the best practices in RoR I am incorporating back into Drupal.