"Sharing common components" is useful, and the fact that any combination of Nix packages is a valid combination is not something that is guaranteed by other package management systems[1].

However there's another, perhaps simpler solution: I can have a separate "wu-ftpd" container, and a separate "hylafax" container. This admittedly might not have been as easy when nix started, but it is very very easy now.

[1]: https://cr.yp.to/slashpackage/studies.html