Why We Don't Really Need Pan-Unicode Fonts Anymore
Several factors have emerged in recent years which, in my mind, have greatly reduced the perceived necessity and utility of the Pan-Unicode fonts.
First, an increasing number of free and liberally-licensed Unicode fonts of very high quality for specific scripts and specific uses (such as scholarly) have been released by dedicated groups and individuals. The existance of George Williams' Open Source FontForge outline font editor has certainly played an important role in the democratization of typography. One need only take a cursory look at some of the excellent work being done by a diverse spectrum of organizations --like SIL International, The Tibetan & Himalayan Digital Library, or the Khmer Software Initiative-- and individual people --like Christopher Harvey's work on fonts for native American languages at LanguageGeek.com, Paul Morrow's work on Babayin at Sarisari Filipino History, or Firefly and Qianqian Fang's contributions to Open Source Chinese fonts-- to realize how true this is.
Secondly, the flexibility of Keith Packard's Fontconfig library allows the construction via simple XML-based configuration files of virtual font sets (such as "sans" and "serif") which can do a better job than any one Pan-Unicode font can at covering the Unicode code space with high-quality glyphs coming out of projects such as those highlighted in the previous paragraph.
We have now arrived at a historic turning-point in the evolution of Open Source digital typography and, in the broader context, of Open Source systems in general. The (typographically-inclined) salamanders have emerged from the depauperate Jurassic morass and, inspired by the fresh possibilities of a new era, a diverse Creataceous flowering of free, high-quality fonts is emerging to fill every niche and ecosystem.
Pan-Unicode fonts may still be useful as fall-back fonts of last resort, but clearly their role in any kind of high-quality typographic endeavour is now much reduced.