Halliday & Matthiessen (2014: 621, 622):
One question that arises in the interpretation of a text is what to do about a conjunction that is implicit. It often happens, especially with temporal and causal sequences, that the semantic relationship is clearly felt to be present but is unexpressed; …
It is perhaps as well, therefore, to be cautious in assigning implicit conjunction in the interpretation of a text. It is likely that there will always be other forms of cohesion present, and that these are the main source of our intuition that there is a pattern of conjunctive relationships as well. (For example, when a lexical relation of hyponymy obtains between lexical items in two successive Themes, a relation of elaboration such as exemplification can often be inferred, as would typically be the case in taxonomic reports.)
Moreover the absence of explicit conjunction is one of the principal variables in English discourse, both as between registers and as between texts in the same register; this variation is obscured if we assume conjunction where it is not expressed. It is important therefore to note those instances where conjunction is being recognised that is implicit; and to characterise the text also without it, to see how much we still feel is being left unaccounted for.