Is ?? necessary, or should you just use the ternary operator (that most are familiar with)
You should use what best expresses your intent. Since there is a null coalesce operator, use it.
On the other hand, since it's so specialized, I don't think it has other uses. I would have preferred an appropriate overload of the || operator, as other languages do. This would be more parsimonious in the language design. But well …
