Halliday (1994: xvi-xvii):
The current preoccupation is with discourse analysis, or 'text linguistics'; and it has sometimes been assumed that this can be carried on without grammar — or even that it is somehow an alternative to grammar. But this is an illusion. A discourse analysis that is not based on grammar is not an analysis at all, but simply a running commentary on a text … the exercise remains a private one in which one explanation is as good or as bad as another.
A text is a semantic unit, not a grammatical one. But meanings are realised through wordings; and without a theory of wordings — that is, a grammar — there is no way of making explicit one's interpretation of the meaning of a text.