I'm glad you mentioned trust, because I fully agree. Trust is the ultimate currency. Anything else is just a substitute. The currency that most closely emulates trust will be the most efficient one, because it will suffer the least from fraudulent actors, the cost of fraud will not be collective, and it will allocate resources in the most effective possible way.

Reputation is a good proxy measure for trust, and serves well to demonstrate why trust is the ultimate currency. Many of us participate in ICO's. Basically we trust the receiving party to act honestly and deliver, not run with the money. A person with a good reputation can get lots of investments, but an anonymous person with no reputation is unlikely to get anything significant.

Reputation makes it easier to trust. I can invest in someone I trust, or lend him money. I can extend some credit to him. The more trust there is in a society, the more it can produce. The less bureaucracy and violence is needed. The more smoothly everything operates.

The opposite can also be seen with the 2008 crisis in the USA. The government backed the loans of people which the banks themselves didn't trust. The government bypassed the thing the whole society is built upon - trust. What happened?

I was really, really excited about Ripple before it decided to concentrate on banks. The reason was that it had a concept that could turn trust directly into capital. Basically anyone could freely establish trust lines between themselves and anyone. For instance I could "trust you with 100€", which would mean that you can pay me with an IOU backed by my trust in you - that you would pay me that 100€ in cash at my will. In effect credit. In addition anyone who in turn trusts me with 100€ could receive a 100€ payment directly from you. The network would grow and everyone would have the sum of all trust credited to them at their disposal, minus anything they already have used and plus anything they have received. In this way I could immediately give you 100€ to use as you wish, without consulting any third party or authority. The implications would have been immense. Just think about the third world with it's capital problems. People are just locked out of the system, even though others have trust in them. I'm hoping Stellar gets this right...

In the case of Ripple/Stellar the currency would be trust, but the unit of account could be whatever you choose.