There are converters available in several places on the Internet that will change SG <-> Brix with the touch of a key. For our purposes, Brix may be considered equivalent to Plato. On brewday, when I'm using my refractometer, all I do is multiply x 4 in my head, and it's close enough. For virtually all homebrewing, the idea that ± .002 - .003 point on FG is going to make any noticeable difference in the finished product is just trying to grind WAY too much point on a pencil.
As far as systems of measurement, there are plenty of historical references on the Internet (and real, paper books) that discuss this. Traditional systems (like the "English" - SAE, etc. system) are based on handy everyday units, what we might call "human." (e.g. human sized)
Science has come to use the metric system, which is very good indeed for science and industrial applications. But there is nothing natural/human in a milliliter or a microgram (as there actually WERE in furlongs and fortnights), nothing much in everyday experience corresponds to these things.....so maybe it shouldn't be used everywhere, despite those who believe it should be imposed on everyone, everywhere.
We believe it's OK to be bilingual, nobody these days would think it strange that I speak both English and Spanish (and enough German, Italian, French, Mandarin & Japanese to order beer), so why on earth would anyone believe that it's necessary to have The One True System of Measurement?
