I don't really want to argue with you since I'm sure you are much more knowledgeable than I am, but are you sure that expressiveness isn't an important quality for a language to have?
I'd say its convenient. Under the hood, most imperative programming languages are largely the same. If you wanted to write a game in Python, at the top most level, it will be structured identically to a game you would write in C or Ruby or something else. The expressiveness will decide whether you can do that task more easily, with fewer bugs. But it isn't a deal breaker.
Much more important, in my opinion, are the language libraries and portability. For instance, I wouldn't write a game in Perl. I could. But I wouldn't. I really like Objective-C and its libraries over Java and its libraries, but when it comes down to it, I can't write Objective-C programs that work on Macs, Windows, and Linus. It's easier to find help writing your Python game in the Python community than it would be to find support writing an Ada application. And of course, there's whatever you need to write in - like if you want to get into the game industry, you'll want to practice your C and C++ even though it might be more fun to write Python. No matter what language you prefer, you'll be using JavaScript for client-side web apps and Objective-C for iOS apps.
At the end of the day, expressiveness isn't about what you can do but more about how easily it is to do something. It makes some languages better suited to some problems and not others, but at the end of the day, you can pretty much solve most problems with most languages. So, don't get upset if you get stuck needing sets in JavaScript.
A language like Ruby seems like it would provide a genuine advantage for certain types of projects.
It is an advantage, but it's not a deal breaker. You can certainly solve those same problems in other languages. But Ruby has Rails, which has been far more of an advantage to its growth than any language feature. Again, it's not what the language can do, but the libraries, community, and necessity.