Halliday & Matthiessen (1999: 290):
For most types of metaphor, it is important to take account of the fact that the metaphor represents more than one construction of goings-on; the metaphor adds a further perspective on the phenomenon being represented, without displacing the perspective that is congruent. […] The grammatical metaphor will typically show features of the congruent perspective as well as features of the metaphorical one.