To be fair, Ethereum is not strictly a cryptocurrency, in the sense that its exclusive purpose is not payments. Sure, coins will have value convertible to fiat currency, but that value will be derived from cost of various applications built on the protocol. The primary purpose of Ether is to be the "fuel" for these applications.
So, high interest in the crowd sale likely derives not from speculation on the value of ether/USD exchange rates, but rather on the value of the cash flow across the many applications being developed on the protocol. As long as developers build these applications, people use them, and they facilitate money changing hands in the form of Ethers, then each Ether will have a set value. Unlike with Bitcoin, consumers will also have a reason to spend Ethers, as they will be the only accepted exchange mechanism of Ethereum applications.
Encouragingly, this will likely act as a self fulfilling prophecy. Investors in Ethereum are motivated to see the ecosystem of decentralized apps develop, so they will build them, and they will use their Ethers. The big problem I could foresee is that because the real value of an Ether will derive from the aggregate value of the Ether "app economy," investors may want to wait before spending their Ethers. But I suspect this will be mitigated by the fact that the apps will only accept Ether as payment.
Ethereum will be successful as a protocol once it gets its "killer app." At that point, there will be no turning back and the value will skyrocket. Surely the market cap will be more than $12.7mm. In fact, by writing this comment I've just convinced myself to invest.