Halliday & Matthiessen (2014: 337):
The ergative model is now fully systemic in English; that is, it is not restricted to certain registers, but together with the transitive model, it makes up the general system of transitivity, and it has been gaining ground over the last half a millennium. The two models complement one another, which is why they are variably foregrounded across registers: they embody different generalisations about the flux of experience, resonating with different situation types.