In response to William Overington's post, it's easier to transcode data from a PUA scheme into Unicode than it is to enter the data from scratch. (The same could be said for a customized ASCII font.) Some users may not wish to wait even the handful of years it took for mainstream Indic complex scripts to be rendered properly.
At this phase of Unicode's progress, however, we shouldn't encourage the interchange of such PUA data. Since it's simple to transcode, any such data should be transcoded prior to interchange or permanent storage. Recipients lacking systems supporting proper Unicode rendering for complex scripts such as Tai Tham could then transcode it to the PUA scheme for display/printing purposes. An OpenType font, a keyboard driver, and a text conversion utility might go a long way towards supporting complex scripts for users whose systems cannot otherwise currently support them. A good keyboard driver should be able to remove some of the burden off of the OpenType tables, enabling multiple fonts covering the same script to be used without having bloated and redundant OpenType tables, by offering some degree of control over the actual character strings which are being stored (and presented to the font for rendering). (Many font developers might consider that any kind of normalization should be handled at input rather than left up to the font. Keyboard developers might have a different idea, though.) A hundred years from now, properly encoded Tai Tham text should be legible. But the ability to display data using temporary PUA schemes which were set up in lieu of proper rendering support appears to fade away over time.
