What's the SQL national character (NCHAR) datatype really for?

Question

As well as CHAR (CHARACTER) and VARCHAR (CHARACTER VARYING), SQL offers an NCHAR (NATIONAL CHARACTER)

Answer 1

Meanwhile the SQL92 standard explains the feature even less helpfully, stating only that NATIONAL CHARACTER is stored in an implementation-defined character set. As opposed to a mere CHARACTER, which is stored in an implementation-defined character set. Which might be a different implementation-defined character set. Or not.

Coincidentally, this is the same "distinction" the C++ standard makes between char and wchar_t. A relic of the Dark Ages of Character Encoding when every language/OS combination has its own character set.

Should one use NVARCHAR for all character (non-binary) storage purposes?

It is not important whether the declared type of your column is VARCHAR or NVARCHAR. But it is important to use Unicode (whether UTF-8, UTF-16, or UTF-32) for all character storage purposes.

Are there currently-popular DBMSs in which it will do something undesirable

Yes: In MS SQL Server, using NCHAR makes your (English) data take up twice as much space. Unfortunately, UTF-8 isn't supported yet.