This was enabled by default in 18a7ce76fc
with the reason that it would be "useful regardless of the desktop
environment.", which I'm not arguing against.
The reason why this should not be enabled by default is that there are a
lot of systems that NixOS runs on that are not desktop systems.
Users on such systems most likely do not want or need this feature and
could even consider this an antifeature.
Furthermore, it is surprising to them to find out that they have this
enabled on their systems.
They might be even more surprised to find that they have polkit enabled
by default, which was a default that was flipped in
a813be071c. For some discussion as to why
see https://github.com/NixOS/nixpkgs/pull/156858.
Evidently, this default is not only surprising to users, but also module
developers, as most if not all modules for desktop environments already
explicity set services.udisks2.enable = true; which they don't need to
right now.
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