Wow, where to begin.
First of all, in-store credit vouchers are not "their own currency" at all. They're just U.S. dollars with restrictions on where they can be spent. They'll always be worth the same amount in dollars because they're not a separate currency at all. other currencies values are tracked separately from each other. Yes, a particular currency can be tied to another currency, but at any point, that tie can be severed. A voucher's tie to the U.S. dollar can NEVER be severed because it IS U.S. dollars.
All cryptocurrencies that follow the same architecture of bitcoin by their nature are controlled by 51% of the owners of the currency. Just because a company creates a new currency and blesses it by accepting it at all their terminals doesn't mean it's controlled by them, as long as it follows the bitcoin architecture.
I was VERY skeptical of bitcoin until I read this article by Marc Andreesen: http://techland.time.com/2014/...
Yes, he has a lot invested in bitcoin companies which gives him a bias and some of his arguments are in my opinion flat out wrong, the majority of what he says does sound very interesting and gave me a new perspective on the currency.