Comment 9 for bug 615435

The current font stack on Linux has no means for requesting non-default OpenType font features, neither GTK/Pango nor Qt support that, so you have to wait the pros and cons of each style; i.e. what is more important having proportional numbers and breaking the look of tables and spread sheets, or making table look good but regular text suboptimal? The number for conformant applications is around zero, there aren't any apart from some modern TeX based typesetting engines. So unless Ubuntu is welling to invest into fixing font rendering stack, depending on OpenType is not a viable option (we have been waiting for what, a decade now?).

And for private use area, no, we should never ever use it for inputting text, this breaks text validity and makes it font dependant, which is a no-no.