I agree that the argument that metals have intrinsic monetary value is rather strained. I tend to believe that the "metal has inherent value" mindset comes from the fact that you can take it anywhere in the world and you'll still be able to trade it for goods and services, which is not true of government-issued notes. That will also be the difference between silver and wampum, but it's entirely empirical; there's no theory that explains why one is inherently different from the other.
> Under such a system, silver would be used as currency because people would need to acquire enough to pay their taxes (and other metals e.g. gold, platinum, copper would not be used as currency).
This is false. Read some history.
> Once upon a time silver was used as currency, and I would say that it is no coincidence that back then silver was a way to pay taxes (among other things).
The history of Chinese taxation (and currency) is quite interesting. I'm only minorly informed, but here are some things that seem relevant:
In what I'll call "phase 1" of the chinese economy, farmers conducted transactions in copper, merchants conducted transactions in silver, and taxes were assessed in kind (grain). This led to major logistical issues in collecting taxes; there was a point where the government announced a tax increase because the ships were arriving at the capital empty. (After a successful effort at reforming delivery, the increase was canceled!)
In "phase 2", farmers still used copper, and merchants still used silver, but taxes were assessed in silver. This was an enormous boon to the logistics of collection, but occasionally beggared farmers when the exchange rate swung against them.
At some point in all this, the government started issuing paper notes. (Notably, these were very common as "reciprocal gifts" to trade missions.) Merchants in the capital would accept these, but silver was far and away the dominant medium of exchange.
It all suggests to me that silver was a way to pay taxes for the same reasons that it was used as a currency, but not because the government needed to have silver available to spend; clearly it didn't. Instead, silver was fairly dense (in terms of buying power per unit volume) and didn't spoil like grain does.
I'm not clear on what you think the difference is between using silver by weight and using silver by face value. The whole idea of a mint is that coins have a standard weight. (Also potentially of interest: over the whole period I've described, China minted copper coins, but never silver ones. This didn't stop silver from being the metal of large-scale commerce and tax collection.)