Wordpress is great for small or digital brochure kind of sites.
I'm not sure what space Rails fits into but I suspect its for those people who want complete and fine-grained database layout and control. This in turn implies you want to store a reasonable amount of data with special requirements on how you want to sort and display that data. Those sorts of web apps are not as common as digital brochure type websites by far.
If I count the number of times I wanted to put up a simple website then I just about automatically reach for wordpress as I know its optimised for most devices out there right out of the box.
If on the other hand I feel the need to banging my head against a brick wall then it would be those times I would reach for rails. I am sure it must be good for something but I haven't figured it out yet.
If I want a simple database web app then I reach for Django/Python but then that's what I'm comfortable with. Its never left me with the feeling of "why on earth did I choose this" as Rails 1.x did to me many years ago. Now Rails has evolved quite a bit since then and I feel like the astonished person that sees how the "green slime" has now evolved into the truly majestic "hairy monster" that stands before him.