Halliday & Matthiessen (2014: 484):
Adverbial conjunctions are as/so long as, as/so far as, (as) much as, e.g. as long as you’re here ... , as far as I know ... , much as I’d like to ... (compare non-finite as well as, which is extending not enhancing). In origin these express limitation, a particular point up to which a certain circumstance is valid.