This. If, somehow, an alternative currency were to suddenly, be accessible anywhere with Square (just as an example of some single vendor that could make altcoins usable at thousands of vendors), it'd be a major reason for people to choose that currency over Bitcoin. It would mean you could buy lunch at a couple dozen places in your home town, pay for services, go see a show, etc. all with your altcoin. Square isn't used in a high percentage of places, but it's in use at a lot more places than Bitcoin. If they transparently added some sort of alternative currency support, it would change the landscape.

Why would a major player back an altcoin? The whole point of my article is that the largest network is the most viable, and so the most logical cryptocoin for any major player to back would be Bitcoin.