This is true, but not all learning experiences need to be shown to the world.
I suppose really I only gave half of an explanation though and did not account for how so many of these posts have gotten voted to the front page. I expect it still has to do with "my favorite language" though.
There's always the possibility that this has real value to some people. Assuming that Notch follows through with his claim that you will be able write arbitrary programs to run ingame, then an online playground to prototype these programs might be very useful. Its appeal is unlikely to be as broad as say, JsFiddle, but putting it out there isn't necessarily just self-aggrandizement.
I didn't mean to imply that it was. But I think calibwam has a point that there have been a disproportionate number of front-page posts today about these implementations, and it's just kind of odd.
