@julian I would say, if mentions and markdown are plugins, then yeah, chats should be a plugin as well. However, they should be a plugin that is shipped with NodeBB, is under github.com/NodeBB, and is maintained by the NodeBB developers.
I'm a strong belief is decentralization. It works with GNU/Linux distros really well. This is why it works:
Say you have a piece of software that isn't modular. After this software is out of active development, a bug is discovered in its chat system. After the maintainer of the code base gets around to it, he finds the source of the bug and fixes it. Since it isn't modular, they can't just push out an update to the chat system. They have to update the whole software, and everyone has to download the large update. Either this, or they have to write a patch script which everybody has to run.
If you have a modular system, sure, you may need to update the plugin every time there is a breaking change, but that had to be done anyways. In a modular system, there can be competition between plugins, which allows the end user more options. When fixing the above issue, either the maintainer can fix the issue in the plugin and distribute it, or, if he's taking too long, another person can fork the plugin, fix it, and distribute it as a fixed version.
If the users of a non-modular forum like the Whatever chat system more than the built in one, they don't have any choice but to request it. The developers probably wouldn't implement the change, because their system works fine for most people.
If the users of a modular forum like the Whatever chat system more than the shipped-with one, they can get somebody to build it for them, and use it in place of the shipped-with one.
That's my reasoning behind using modular software.