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.

The whole area of symbols, denominations etc is a confusing mess right now, it opens up the potential for mistakes and makes Bitcoin look unprofessional. Part of the reason I don't want us to revisit this at the moment is we need to grab onto any consistency we can get. People want to think in terms of a single unit. BTC vs mBTC is already bad enough, it'd be easy to miss the denomination and do some sums wrong. Introducing a third unit, especially one that skips the intervening nanoBTC, seems like a way to make mistakes even more common!