What do you hold when you hold USD in a bank account? The best argument I've heard for the value of USD is:
1. is that you can pay your US federal and state taxes in USD
2. and many companies and individuals will accept USD in trade for goods and services.
Measuring BTC by that same rule you'll find:
1. You can pay your Ohio state taxes in Bitcoin [0],
2. and you can trade BTC (Bitcoin) for some goods and services (overstock for example).
Thus if USD has value by the measure above than BTC must share some of that value. The value of BTC might be significantly smaller than USD but it must be non-zero.
>When you hold stocks you hold actual ownership of a productive corporation
Not every company is productive. The stock is worth what someone is willing to pay for it now or at some future point. Companies have issued stock as cryptocurrency tokens.
>When you hold Bitcoin...you don't hold anything which can convert into something useful at the end of the day - you hold the useless result of expended computing power
You raise an interesting point here. Should currencies have a use other than being a medium of exchange? For example you can burn paper money to generate heat or melt down coinage into useful metals. Does having that property make paper money or physical coins better as a currency? Goldbugs would say yes, I'm less sure.
[0]: https://www.wsj.com/articles/pay-taxes-with-bitcoin-ohio-say...