Another way to look at it is - ideas are only a starting point for a game development journey. Ideas usually do not survive the game development process unchanged -- they change a lot during development as you test and refine them.
The job of a professional game designer is not to simply come up with game ideas and then go home -- a designer has to babysit the idea during the entire period of development, which can be years. During that time they're refining ideas and adapting them to the unforseen. If you want to be a game designer, iterating, modifying, nurturing ideas and adapting ideas to constraints is your real job.
If you have an idea for a great MMO it has zero value to you, because you do not have the capability to act on that idea. It costs millions of dollars to make a simple game and hundreds of millions to make a ground-breaking blockbuster game -- so ideas for big games are worthless to people who don't have that kind of money behind them (And before you say it - it's pretty much impossible to sell a game idea. So it still has zero value to you).
So, an idea for a big game only has any value for the people who are actually in the business of making big games... and these people do not buy ideas...
So, if you want to ever design a big game / MMO / etc and have it be made, you have to get hired as a game designer at a company that makes big games / MMOs / etc...
So, if your goal is to get hired as a game designer, you need to get as much practice in designing games as possible...
But, if your designs for MMOs will never get made into games, that's a catch-22 situation, right?
Nope, you can get practice by sharing and discussing your ideas and learning from them!! :) That is how you find value in ideas that you don't have the capability to act on.
And if someone does actually "steal" one of your ideas and turn it into a big game, then that is actually creating value for you. If you've previously published the idea, and then it turns up in a big game afterwards, then you can claim credit for inventing it and use that of evidence as to why someone should hire you as a game designer.