What you should note is that most WP plugins that integrate with external services usually just insert code in the the views/themes. Also worth mentioning that there are a lot of features in WP that you will never use or need and a few features that you will integrate but may also need in your rails app(AB testing is the obvious one).
Like I mentioned earlier there are easy workaround's for most features that you would need:
1. Blogging -> I'd go static unless I was doing more than 8-10 posts a month. Other options include Jekyll, Octopress, Ghost or Rails/Sinatara.
2. AB Testing (You may want this for your rails app too) -> a gem[0], web apps[1] and see what other are doing[2][3].
3. Sales -> I am guessing this is mostly going to be lead generation forms and such, depending on what other services you use it is extremely simple to include these via html, js or iframes.
4. Social Media -> For users to share you could use something like www.sharethis.com there are a few alternatives too
5. Comments -> Disqus, take 30 secs to setup and install.
6. Permalinks -> Easy to do with config/routes.rb
[0] https://github.com/andrew/split [1] https://www.optimizely.com/ | http://visualwebsiteoptimizer.com/ [2] http://37signals.com/svn/posts/2977-behind-the-scenes-highri... [3] http://37signals.com/svn/posts/2983-behind-the-scenes-ab-tes...