Halliday & Matthiessen (2014: 300):
They are the principal types in that they are the cornerstones of the grammar in its guise as a theory of experience, they present three distinct kinds of structural configuration, and they account for the majority of all clauses in a text (‘material’ and ‘relational’ seem to be roughly balanced in frequency over the language as a whole, followed by ‘mental’, although the pattern varies considerably among different registers).