1. built-in blogging ... everyone and their brother has built a blogging platform for Rails. WP's blogging platform is a one-size-fits-all approach which usually means you only use about 10% of what it's capable of, so having all the extra cruft is just unnecessary. You want a simple blog in Rails?
rails g model Post author_id:integer title:string body:text
2. integration with email marketing – the plugin is only going to get you half-way there. On either platform, rails or wordpress or anything else, you still need to build the component that will take your specific business data and turn that into actionable emails. Example, email users who joined in the last 30 days who have not used our service. That is something you need to build yourself for your specific product no matter what back-end you are using.3. Sales-funnel and AB testing – Mixpanel or Google Analytics will give you the funneling. AB testing can be done with your own solution to solve your exact problems, or check out the variety of gems that help make it easy: https://www.ruby-toolbox.com/categories/A_B_Testing
4. Social media marketing ... again throwing the idea around that you can click "install plugin" and have all your bases covered here is not very realistic. What do you mean by this? Automatically tweet or post to fb based on certain events on the app? Auto tweet/post your blog posts?
Either way, I'd argue against doing anything with Wordpress other than a static blog.yourdomain.com. You can rebuild ANY feature or plugin in the WP domain very easily in Rails.