We would prefer, for testing purposes, that there be at least one TZ identifier for every ISO country code, even uninhabited rocks like BV. However, that principle doesn't extend to the reserved codes, which have no UN M.49
numeric code (the basis for the ISO codes), and in many cases are reserved only by historical accident -- it doesn't mean that they will ever be used as country codes. Cf.
http://unstats.un.org/unsd/methods/m49/m49alpha.htm
Note that Serbia and Montenegro have also been given separate UN codes since their recent split; ISO lags, however, in assigning them alphabetic codes, so it is unclear how long it will be until they are assigned.
Mark