If plugins are optional, then the modules that are only needed for plugins, are optional as well.
The opinion of your average distro packaging standards, is that plugins which aren't necessary for core functionality, are optional extras.
That's why its nice to list them separately, and if a distro prioritizes user-friendliness, they can default to installing them, and if a distro prioritizes giving users the tools to make unusually informed decisions, they can do that too.
And for a standalone binary download, it makes sense to include them as a matter of course.
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More information makes it more likely that packagers include those modules in any shape or form, rather than having some maintainer somewhere say "huh, this isn't doing anything, let's ax it".
