precious metals, but most especially gold, have always been prized for hard currencies. Always will. Specie currencies have always been the recourse of frontier areas.
That being said, however, softer currencies (paper, symbolic tokens, etc) have been used almost since the dawn of currency, if not causing the dawn of currency.
The truly soft moneys (Electronic funds, checks, etc) have grown out of currency and merchant houses.
So, since the metal itself is of both commercial and artistic value, gold coins will probably remain a medium of exchange for a long time. Even now, some gold coins are circulated, albeit not with face values, in payment of debts.
And UL is confusing paper monies with the truly hard currencies of precious metals. I suspect many local coins have high denominations in traveller due to being made of Lanthanum (which is used to make jump drives), Gold, and probably somem other non-radioactive metals.
That cash, however, represents a promise of value made by the imperium. It makes perfect sense in a setting where goods can move faster than information. They are hard to forge (TTA sets some really high standards, IIRC forgery 3+ to even attempt a credible forgery; can't find it to check), and require specialized, expensive, and trackable equipment.
Then again, I've held $10,000.00 in cash. It's fairly convenient.
Forgery will occur. Even truly hard currencies get debased and/or clipped. it is, however, simply a matter of faith: is the general perception that the issuer is:
1) capable of making good on repaying the value
2) capable of keeping forgeries down to a minimum
3) capable of enforcing its will in the areas accepted
4) capable of disbursing cash in sufficient quantities and with sufficient speed to get the flow started
The Imperium, probably by accepting only Imperial Credits as payment for the Imperial Taxes, and paying only in imperial credits, usually in cash. This is, in fact, part of how the US made the Federal Dollar the currency of choice: you had to pay your taxes in US dollars.
The forgery issue affects all currencies, most especially electronic ones. SOme have argued that electronic currencies are in fact the most subject to both fraud and forgery, as electronic money is easy to fake...
The ability to enforce its will over the areas in which it is intended to be spent affects both forgery and creating demand. If a government can say "You will only pay us in Zlotniks, and you can only get zlotniks from us or our minions... and the penalty for failure to pay the assessed tax is ...", if you can't live up to the threat, your currency is meaningless too. If you make that threat, and live up to it (See also the Whiskey Rebellion), your currency will be accepted, for fear of the alternative.
If you can't get sufficient cash to the place of payment, then it's meaningless as well, as you then have the issue of scrip or promissory. Neither is as well received as a primary currency.
Cash is no luxury. Credit cards, checks, and electronic payments are luxuries... ones which you pay for by otherwise usurious interest rates on due amounts, annual fees (both consumer and merchant), and % of sale fees to merchants, and lack of interest paid on deposited amounts. All those fees are to make up for the much-higher-than-cash forgery/fraud/deceptive-practice losses and high rates of default.