For the majority of people, a string format that accepts 100% of ASCII but 0.1% of Unicode is just as useless as one that only accepts 95% of ASCII. Therefore the goal should never be to get your ASCII coverage from 95% of 100%.
Resolving the Unicode issues is undeniably a higher priority for speakers of foreign languages, but there are still plenty of languages and libraries whose support for Unicode ranges from nonexistent, to antiquated, to limited, to just plain broken.
There's nothing application developers can do in the short term to fix those problems, but by examining their own code and removing fallacious assumptions they can better facilitate the proper handling of Unicode once it becomes available in their underlying technologies. In the mean time, though, they may only have ASCII, or ISO 8859-X, or KOI8-R, or Shift-JIS, etc. to test with.
The first A in Ascii stands for American. This is because it was an American standard, created by and for Americans. Who pretty much all speak English (and spend dollars $!). What about ascii isn't anglocentric?