My take on Tilman's suggestions was that using ns_choosecharset to determine the charset would complicate troubleshooting character display problems down the line... but that's just a hunch.
It would be nice to know what percentage of browsers hitting Planet DO accept utf-8, as well as what other charsets are accepted for Asian locales that potentially use different charsets (for example, shit_jis and euc-jp are used in japan). I wonder if this information can be deduced from browser stats, or if it would be worthwhile writing a bit of code that asks browsers for charset preferences and records them (along with client IP and locale) in a table?