Right but then you destroy the host's hard work. They spend time coding something unique for their server only so other people can use it and rip it off? I find that quite unfair. If someone wants to share their gamemode or add-ons then that's their own business.
Glass didn't address this point; What happens when the host of that server stops hosting, whether they get bored or go on to do something new?
Do they still keep those add-ons private? Even if they're very useful for everyone else? Even if people still want to play on the old server with the old add-ons?
If you do keep them, then to what benefit? You certainly aren't using it, you might even forget it exists (in which case all your hardwork in making it goes to waste). It might mean you have more people at your server if they come back to see if you're using it again, or to beg for it. But what does that do, bar inflating your ego. People will eventually stop bothering to beg for it, and having it to yourself won't benefit anyone.
So, why not release it? You don't lose anything from doing that. You were hardly going to be able to monetise it by keeping it to yourself (seeing as most of the community are quite young and aren't likely in a position where they can buy an add-on, and most purchases for add-ons come from people designing and requesting a new add-on, not buying someone's old tat).
If you release it then there can be hundreds of other players with the add-on. Players who can use it to recreate your server (again, this hurts no one if you're not even hosting it) or create their own unique servers and games with it.
In fact, I personally would consider that an honor to have my gamemode hosted by others because they think its good.
This should also be one of the big motivations to give away your stuff. The lack of this notion in the community is what can spoil it at times.
Whether you're creating an add-on, or a gamemode, or a build, then you should surely feel honoured when people use it and enjoy them. What's the benefit of creating things for this relatively small sandbox game if no one (not even you) are using and enjoying them?
There's a big fear that other people will take credit, but that really is preposterous. All it takes is for you to be public with what you make. A big ol' topic on the forums sharing your creation and giving it away is the easiest way to make sure people know you made it.
Yeah, there will be people who still take credit (particularly with builds since anyone joining a server can save anything they find), but most everyone will know it was you when you have a topic showing so. And if someone still refuses to believe you, or continues to claim ownership, then forget and forget them. Their opinion and lies aren't important. The majority of the community will believe you if you've got proof on the forums.
There really is no reason not to share in this game. It'll only ever mean that other people can continue to enjoy your hard work.