Using milli- and micro- notation for currency units is also not very
well supported. Last time this thread was active, I believe there was a
suggestion to use 1 XBT == 1 uBTC.
Unfortunately I think some people already started using XBT to mean the same as BTC (another ship that sailed: somehow Bhutan will have to live with it). So if some software started to redefine it to mean something else, that seems like a recipe for accidentally sending far too much or too little money by mistake.