I guess the danger is similar to commodities, where the value may start to be driven by people just trading it with no interest in using it, rather than intrinsically.

My memory is getting tested here but I think the other reason its gone up so much is that the supply is limited by how many keys people can create, which as I said is getting more and more difficult each time. So while technically the supply is infinite, I guess it follows a log sort of curve.

There is no requirement for the currency to exist, as Dunster said. And yet it is growing in popularity and if it continues to have advantages over traditional currencies, I see no reason why the demand for it would vanish.