Having good blogging software is not as simple as generating a data structure.
There's plenty to be said for having a good admin UI (that non-developers can easily figure out). The flexibility and depth you get out of the box is not going to be matched by anything you can do yourself in a short amount of time.
> You can rebuild ANY feature or plugin in the WP domain very easily in Rails
This is just false - there's so many well matured products in the WP ecosystem that would take way too long to quickly develop no matter what language you're using. There's a lot of plugins built specifically in mind for a marketing / brochure type website that have had years of development that you can benefit from immediately (SEO plugins, contact forms, ecommerce..)
I'm not suggesting that WP as a frontend is the way to go, but I would not throw it out with the assumption that you will be able to rebuild it easily.
I would weight the decision entirely on how much integration will need to be done and what your marketing needs are. A couple of questions I'd ask are:
1) Is there going to be someone constantly tinkering with the site to tweak SEO as much as possible?
2) Will content be frequently updated, or will the site act more as a landing page with just occasional updates?
3) Do you ever need to authenticate a user on the marketing site? (I find cross-application authentication to be one of the harder integration problems to tackle and a good reason to not use a separate CMS system)
If it's just a landing page with only occasional updates, building it out as a simple view within your existing app framework may work just fine (or heck, just some static HTML files may fit the bill).