title: Is it still useful to declare development dependencies in a gemspec when using bundler?
body: <p>I am working on a new Ruby Gem. I am familiar with using Bundler to manage gems:</p>
  
  <pre><code>source "https://rubygems.org"
  
  gemspec
  
  gem 'rspec-rails'
  </code></pre>
  
  <p>I am familiar with specifying dependencies in a gemspec file:</p>
  
  <pre><code>Gem::Specification.new do |s|
    # ...
    s.add_dependency "rails", "~&gt; 4.1.5"
  end
  </code></pre>
  
  <p>The Gemfile that was generated mentions that I should move my dependency declarations from my Gemfile to my gemspec when I'm ready to release. </p>
  
  <pre><code># Declare any dependencies that are still in development here instead of in
  # your gemspec. These might include edge Rails or gems from your path or
  # Git. Remember to move these dependencies to your gemspec before releasing
  # your gem to rubygems.org.
  </code></pre>
  
  <p>Why would I want to do this? Why should my gemspec care what gems I'm using in development? What purpose does <code>development_dependency</code> serve that Bundler doesn't already do for me?</p>