Halliday & Matthiessen (2014: 262):
This tells us something significant about a prototypical configuration of ‘being’: the experiential ‘weight’ is construed in the two participants, and the process is merely a highly generalised link between these two participants … Thus the verbs that occur most frequently as the Process of a ‘relational’ clause are be and have; and they are typically both unaccented and phonologically reduced … This weak phonological presence of the Process represents iconically its highly generalised grammatical nature. The limiting case of weak presence is absence; and the Process is in fact structurally absent in certain ‘non-finite’ ‘relational’ clauses in English … and in many languages there is no structurally present Process in the ‘unmarked’ type of ‘relational’ clause … Here the ‘relational’ clause is simply a configuration of ‘Be-er1’ + ‘Be-er2’.