They do have intrinsic value, but it’s largely incalculable at the moment as it depends on value in use calculations that, in turn, depend on assumptions with respect to adoption rates, turnover and so on. That said, i suspect the majority of those presently speculating in crypto currencies neither cares about nor has any idea what “intrinsic value” is.
The other critical assumption taken for granted is the fixed supply of a given crypto currency, which gives the hypothetical rarity/store of value. That, to me, is the biggest fallacy of all - i’m pretty sure man can find a way to make more Bitcoin, Ripple etc. if he’s sufficiently incentivised to do so, and human nature has a pretty reliable track record stretching back many millennia of responding to monetary incentives as you’d expect.