The plugins are not the same as the IDEs. For example PyCharm doesn't distinguish between project settings and IDE settings the way IntelliJ does. The UI for choosing your python version and virtualenv is baked right into the IDE settings where as that is not the case when you install the Python plugin in IntelliJ. The .idea directories also do not seem to be compatible across RubyMine and IntelliJ. I have mistakenly opened up the wrong project with RubyMine and it scrambled the .idea for a non-ruby project.

The same is true with WebStorm, the UI and features found in WebStorm are not the same as in the JavaScript plugin. You cannot buy IntelliJ IDEA Ultimate and get all the features of all the other IDEs, you need to have separate licenses.