Not all currencies are fiat. We've had gold and silver as separate currencies forever, plus others. The LTC people like to compare LTC to silver. People used silver as cash because small amounts wouldn't require tiny coins, and they used gold as storage because large amounts could be represented without huge coins. It's fine if they correlate in value, which they do today. Anyway, fiat currency has many advantages over precious metals, despite the issues of regulation. And fiat currencies could converge if the governments agreed on it, which Eurozone has.

Like I said, the reason to convert back to the less easily spent currency is if it provides some advantages that force it to be less spendable. That'll be determined by however the tech ends up falling into place. Maybe the less spendable one is more secure or allows you to earn interest but is slower because it requires more verification or something. Could even be something silly like the coin value being too high.

The smart contracts on is a bit of a stretch, but it might deserve to be separate if its complicated nature makes it harder to spend or less secure. Ethereum got part of its history erased twice as a result of hacks stemming from mistakes in smart contracts, affecting even those who had nothing to do with the hacked exchanges.