This is somewhat of a fuzzy area in JPA. The spec seems to require a discriminator, some JPA providers require the discriminator, and some don't.
In general it is better to have the discriminator, as it is easier to determine the class for each row by checking the discriminator, instead of having to null check each subclass' table's join columns.
Having a discriminator also makes querying much easier, for example query all instances of two or more subclasses. It also allows for inheritance to be mixed, i.e. some subclasses use joined and some share their parent's table.