Keep in mind that the $300 million market cap isn't really a great representation of a cryptocoin's value since this can be manipulated if there is low liquidity - keep in mind that STEEM is only currently traded on one exchange.
Also, I would say that there is a sort of multi-level marketing aspect to this whole operation and while I like this concept I am a bit skeptical about the rewards model this site implements. Certainly all those posts earning hundres or thousands of dollars can't cash out right away or the value would plummet immediately which is why the devs needed to create the concept of Steam Power which prevents one from getting their reward immediately by promising greater rewards for holding.
I am skeptical too. Lets say the initial offering of Steem had not been bought into at all by the bitcoin community. Then Steem would only be worth something inside this website. It would more accurately represent the value of the content we're creating here. Blogging on the internet isn't worth much. But we did buy into it and in doing so gave it a perceived real world value. To justify that valuation something of equal value must be created here.
Ya, I hesitated to call this multilevel marketing or a pyramid scheme because I don't completely understand it yet. Having its own currency makes things complicated. But it certainly has aspects of that.