I don't remember when they were put back into the "master". They used to be more separate.
Wrong. Plugins developed and maintained by the Roundcube developers always have been part of the master git repository together with the code code. The planned composer-based plugin repository is meant for 3rd party plugins. The main goal of that repository is to allow plugin developers to publish their modules on a centralized platform and allow Roundcube sysadmins to pull them into their local installations. That isn't necessary for core plugins because they're already part of the distribution package.
Arguably it would actually be nice if the core plugins would also move to composer. Then you can easily do a composer update and grab updates of core plugins as well.
I am doing a lot with a php framework called Laravel lately, and I really like their philosophy regarding composer. If you're going to use composer, REALLY use composer. Next step, move all of roundcube to composer and split out core modules into composer modules. Also, maybe re-use other third-party composer packages in roundcube. For this end, roundcube should become PSR-0 compatible :)
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