Halliday & Matthiessen (1999: 545):
… one effect of grammatical metaphor is to render many of the semantic relationships implicit: if the happening is construed as a clause, the semantic relations are spelt out in the configuration of grammatical elements, whereas if it is construed as a nominal group they are not, or only partially so … On the whole, the greater the degree of metaphor in the grammar, the more the reader needs to know in order to understand the text.