The problem with open servers, is there is nothing to prevent griefing, theft or harassment, unless the server is VERY well and VERY actively moderated. Characters are not persistent on the server, so someone could hop on, take everything you own, and leave, with no trace.
There is nothing to prevent someone from spawning in a bunch of materials in a single player world, jumping on the server and dropping it all down.
This makes it so that outside of your own personal progress, the only real thing to do on a server is to build, but with no built-in tools for managing a community, it's very much like an anarchy minecraft server - Anything goes, build at your own risk.