Why critical linguistics? and why Halliday's systemic functional linguistics for that? Listen to Roger Fowler!
'...The prevailing orthodoxy of linguistics is that it is a descriptive discipline which has no business passing comments on materials which it analyses; neither prescribing usage nor negatively evaluating the substance of its enquiries. But I see no reason why there should not be branches of linguistics with different goals and procedures, and since values are so thoroughly implicated in linguistic usage, it seems justifiable to practise a kind of linguistics directed towards understanding such values, and this is the branch which has become known as critical linguistics. That is the method followed in this book. Now, the word 'critical' could be intended, or taken to be intended, to denote negative evaluation, but this negativity is not necessarily the aim of critical linguistics. As far as I am concerned, critical linguistics simply means an enqiry into the relations between signs, meanings and the social and historical conditions which govern the semiotic structure of discourse, using a particular kind of linguistic analysis. This activity requires a very specific model of linguistics. The model has not only identify, and to label reliably, certain key linguistic constructions; it has to relate them to context in a specific way. The familiar transformational-generative linguistics invented by Noam Chomsky provides my eclectic model with some descriptive terminology, but is in general terms unsuitable, because its aim is to refer linguistic structures to the set of structural possibilities that are available to human language as a universal phenomenon, presumably genetically programmed in the human brain; Chomsky is not interested in the role of language in real use (and indeed will not allow such matters to be a valid concern of linguistics). Halliday's systemic-functional linguistics, on the other hand, is speciffically geared to relating structure to communicative function, and this model provides most of my descriptive apparatus....'
( Roger Fowler. 1991 Language in the News: Discourse and Ideology in the Press. London/ New York: Routledge. p.5)