Question: It seems like a strange feature that rubygems wants to know what my development dependencies are, so far I never saw it used to actually load these when running the gem tests.

Answer: 
If someone decides to hack your gem for whatever reason, e.g. they fork it on Github to add a feature to it (which they might want to contribute), it helps if they know what development dependencies your gem needs (e.g. testing frameworks, mocking tools etc.).

If you're using bundler with the `gemspec` command, it will hook into your gemspec dependencies and install the development dependencies along with the runtime ones when you run

```
bundle install

```

This saves you having to install these gems manually. 

The gem command can also list all the dependencies of a gem including the development ones:

```
gem dependency my_gem

Gem my_gem-0.1.3
  activerecord (~> 3.0.0)
  json (~> 1.4.3, development)
  rake (>= 0, development)
  rspec (~> 2.5.0, development)
  ruby-openid (~> 2.1.0)

```

Once again this is probably more for other people rather than for yourself.
My gems have normally unit-test. This test requires sometimes gems, which are not needed to use the gem. Or perhaps you need additional gems to generate parts of the gem.

---

Once I misused development dependencies to define 'optional dependecies' (dependecies were necessary for some specific features of my gem, but not needed for the 'normal' usage).

Example:
My application offered the posibility to export to a file as text or pdf. The pdf-generation uses [prawn](https://rubygems.org/gems/prawn). So prawn is a dependecy - but it is not necessary to use the application, only a specific feature needs it.

So I didn't add prawn to gem dependencies (it is not necessary), but to the development dependencies (it is usefull for the gem).