To address your question, If we are to use Bit-coin as an example, it seems Bit-coin is only valuable because of its convertibility to US and other currencies which themselves operate within the framework of capitalism , Which I addressed earlier.
If I may, I would like to understand that your question is this,
Does an item has to be both desirable and scarce in order for it to retain value?
I believe not, for much of history human credit was extended, many people in villages and local communities still do, for example in India and even rural America; shop-keepers will keep an ledger or an tab, (extending credit to locals and regulars).
This is simply the individuals trust or credit in another, and this is infinite in quantity and desirable and in my opinion, has great value.
My question to you is as follows,
1) Do you believe that humans, are inherently self-serving?
2) Do you believe that given the chance, humans have an infinite appetite for consuming?
According to modern economic thought, because every individual seeks to gain advantage for his personal benefit, the market is the fairest way to distribute the scarce resources.
If I think of anything more I'll edit this post, till otherwise peace
1) Yes it is a basic instinct, called survival
2) Maybe not everyone, but enough people would take everything they wanted, which would create an extreme shortage
Questions/points for you:
1) Would you work hard and consume less than everyone else? Or would you consume more and work less than everyone else? --if everything was free. most people would barely work and just consume
2) No merchant would accept these "points" as they could be infinitely printed. Unlimited supply and low demand = worthless
3) Bitcoin is not valuable because it can be converted to USD. It is valuable for its portability, divisibility, decentralized, limited supply, security and many other properties. It bitcoin becomes the currency of any country or the world it will be independent of the USD and prices will just be listed in bitcoins.
4) Remember that people were spending USD in computer mining + electricity long before bitcoin was even worth a single penny.