If the force repacks option is enabled on the server, all resource packs are prevented.  The expected behavior is that the server packs should sit onto of any packs the client has, the same way client packs sits on top of the vanilla files.

If the force option is not in place, then clients are able to use their resource packs.

Having the ability to override certain files (such as stone textures to help prevent X-ray, add additional blocks/functions, or hide seed GUI to prevent looking up locations, etc) is critical for server communities.  Requiring players to use it shouldn't prevent non-effected files in packs from being used.