Have you ever tried converting from one system to the other in your head? It basically can't be done. Conversion is an easy process if you're willing to use pencil and paper, but at that point you're most of the way to using arbitrary conversion factors.
I sometimes do between long and short scales (English uses one, Polish uses the other one). So to convert from Polish "bilion" to English, you go "bilion" -> 10^12 -> "trillion". Or, in the rarer case, "sextillion" -> 10^21 = 10^3 * 10^18 -> "tysiąc trylionów". Adding or subtracting the exponent of 10 is much easier than multiplying by random numbers (e.g. 5280 to get from miles to feet).
