One good reason might be if you think the value is going to increase beyond what you paid for it, like people paying whatever mad amount for bitcoins.

I think people who spend huge amounts on diamonds are stupid, but everyone has their reasons.

There are loads of ethical systems that condemn wastefulness of money; the US has had at various points at least a plurality, if not a majority of people ascribe to moral systems derived from puritanism, so at least in the US, it is unsurprising to hear someone saying that wasting money is unethical.