Indeed but, as TTC implied, the electrician may decide that it would not be appropriate to export the earth, in which case there would be no logical sense to using 3-core (unless, of course, it were cheaper!).It does not have to be 3 core. But where two core is used and the armour is used for earth then the cable needs to be tested to ensure the plastic coating is not damaged as even a small hole can cause the steel to corrode and so lose the earth. So it is common to use belt and braces so if the SWA does fail the earth is not lost. On long runs it also helps keep the ELI down.
Kind Regards, John