The system has other advantages: once you know how it works, you only need to remember one size to be able to reconstruct the others (or, you could do it from scratch because of the "A0 is 1 square meter" thing, but this means A0 is 2^(1/4) by 1/2^(1/4) meters, so most people would need a calculator for this). I learned the 21cm x 29.7cm bit when I was about 12, and never forgot it.
But yes, it's all part of the same metric system (though I'm not sure it was designed during the French revolution, like the rest of the metric system - probably not). Like the "we want the unit of length to be common to all of mankind, so we'll base it on the length of a meridian" bit. The only failure was that the USA failed to adopt it; that the British didn't was to be expected, but come on, at the time the USA were clearly friendlier with the French than the Brits, right?