Isn't the worth of any crypto-currency down to market psychology though? By that metric any price movement is irrational.

Economics is a social science. It could be argued very few things have intrinsic value. It could be argued all markets are down to purely human psychology.

Birds, for example, don't really care how much you paid for those 3000 tonnes of wheat, only that there's a hole in one of the train carriages transporting it.

That's mostly true, but it doesn't completely work. 200 of something with a low market cap with the potential to gain 200-4000% is way better than 0.00159 of something with a large market cap and low likelihood of doubling. E.g. a random altcoin vs bitcoin today