That's a flawed argument, because you can by the sandwich for $5 worth of BTCs, and then immediately buy the BTCs back with cash.
Of course, that seems excessive for the individual user, which is why your wallet service provider will do it on your behalf and assume the risk. I imagine Coinbase will do this very soon.
Assuming that in a couple of years there will be a functioning derivatives market, the wallet service provider can then offload that risk to investors.
Speculation isn't a "bad thing", it's a natural thing that is necessary for an efficient market.