As a lower bound, it's at least as useful as other things that people regard as valuable mostly because, historically, people have regarded it as valuable. Gold is in this category -- but so would be Awesomium, a substance I just made up but which myself and my associates are certain is gonna be the Next Big Thing, so get in now before it gets big, YOLO!

If you trust institutions like banks, tax departments and reserve banks, and transacting with them is easy and cheap, the value of crypto as a medium of exchange is very low. This is how the majority of people feel in the developed world, though whether this is rational (given things like the fact a government can just unilaterally change the rules by printing, or burning, a bunch of money) is disputed. To the extent you don't trust these institutions, or they make transacting hard, the value of alternatives, like cash notes and coins, barter, or crypto increases.