A server can always place links for the mods it uses in their description or rules pages or wherever else so people can download them. In most cases, these should be linked directly to the original mod locations and not made downloadable from other sources because of the reason BFT2020 mentioned and also so that people can easily find updated versions of those mods. If a mod author gives permission to make them available from some other source, then that is different, though it's still not a good choice, imo.
Once Workshop is made available in 4.0, it will make it easier to download and install the mods. Just need to wait until then.