I live in Mexico, where the symbol for MXN (Mexican pesos) is $, same as USD. In everydaylife, this causes no problem, since there's no ambiguity: everything is written in pesos, you pay in pesos, you charge in pesos. From time to time, though, there's an ambiguity about whether the price is in USD or in MXN, especially in "international" zones (airports, touristic places, a whole big part of the Internet, etc.). When I first arrived at the airport, I found the price for losing one's "forma migratoria" ($80 or $100 IIRC) very high, until I realized it was Mexican pesos. Also, on an art website I'm doing (
http://plaztika.com), prices are in Mexican pesos, and I can't use the $ symbol because it would be ambiguous. Some French artists will join the website soon and will have prices in EUR, and in that case I'll be able to use €, but for prices in pesos, I'll have to stick to "MXN", which means concretely that the $ symbol cannot be used. Same for any future artwork sold in American dollars, I'll need to use "USD" or "dollars". Sad but true. I've been hating this long before I heard about Bitcoins, so this is not a tweaked argument.
The same situation could happen if ฿ were used for both BTC and THB (Thai Bath): it will work as long as there's no ambiguity (e.g. here on this forum), but for example on that art website I won't be able to use it ever.
So it's fine by me if people start using ฿, but it will be only a partial feature. While it's still time to choose, let's be more clever than that.