Rails is almost entirely a wrapper for these components like ActiveModel and Sprockets and such. You can use them apart from Rails but that's why they are all on the ports tree. It's probably the only thing that uses them.
This is very much not true for many of the components. I have a few gems that rely on ActiveSupport and ActiveModel, Sprockets has integration with other frameworks, etc.
That said, you're right, the 'rails' gem is really a meta-gem that installs all the right versions of the other ones.
Yeah but are those other gems in the OpenBSD ports tree? The point is not that they are not used for other things but that they are probably not used in OpenBSD