Halliday & Matthiessen (1999: 466-7):
The difference between the two perspectives has been obscured for two reasons. In the first place, syntacticians have continued to tend to fail to recognise the unity of the verbal group, favouring instead a Predicate-based constituent, the Verb Phrase, with the Auxiliary then detached within it or located outside it altogether; furthermore, parts of the verbal group complex have been exported to a higher node (e.g. seem, continue). In the second place, the preference for immediate constituency rather than rank-based models of constituency effectively masks the generalisations that can be made about any one rank.