I guess I don't really have an opinion about that. Certainly an exact representation of decimal numbers was essential, and was something we needed to implement at the time, but going to a fully rational numeric stack was arguably overkill.

The current value of held assets in another currency isn't really "counting" any more, it's a prediction of the outcome of some future transaction that hasn't happened. So I'm less concerned about exactly computing it than I am in never making a mistake in assets that I am counting, i.e. keeping in one account and only incrementing and decrementing the amount when a transaction occurs.