In a some countries national ID numbers are good primary keys, but that's only when they're designed (and used internally) as such by governments. e.g. Finnish, French, or Israeli ID numbers. Even then, though, using them limits you to people who are in the system - no dealing with international customers, for example.
At least in Finland, national ID can change in some cases, e.g. when you change your legal gender or if it's repeatedly misused by someone else (like in identify theft scenario).
