You've been around this forum for a bit, so i'm sure this conversation happened more times than you can shake a stick at. But i''ll bite: You're talking about potential, not current, usefulness -- am i right? Because right now, if i don't need to gamble or buy from SR, my fiat does just fine, while Bitcoin ... well, i can't even gas up my car with it. As far as potential usefulness, I suggest you get behind my Bitcoin V2.0 -- I'll let you in on the ground floor & toss you a few coins.
Oh, and you can mine it while the difficulty's still low (sorry, old Bitcoin -- you had your day in the sun). My 2.0 is no different from bitcoin, other than lower total coin count (+1 for RARITY), so it's potential usefulness is no different from Bitcoin V1.0.
So, is it rarity & usefulness that gives Bitcoin value, or could it be ... something else?
Last week I ran a poetry contest. The entries came from all over the world (I can only assume, I did not actually ask where anybody was from). At the end of the contest I was able to send a little bit of money to about ten different people I had never met before and all I had to identify them was a single string of characters, the money was sent instantly for an a "generous" fee of the equivalent of 0.1 USD. You don't see how bitcoins are useful? Maybe I could have sent them cash in the mail ... 0.41 usd
each for the postage, but wait I don't have addresses for them, I could ask for their mailing address, but no, they would not want to give me (an anonymous guy on the internet) that information, so my giveaway would have failed.