I join this discussion late. I have read research that observes
where pauses occur in speech and writing (sorry, this was years
ago and I don't have citations; just remeberance). The pause
breaks between clauses, interestingly, take place AFTER the
subordinating conjuntion, not before, suggesting that the brain can
plan only a clause plus a direction (the direction comes from the
conjunction) at one time. This constitutes a limitation on
consciousness or on the unconscious sentence planning process
(I can't remember which). The point is, when a student puts a comma
after "but" instead of before, that student is perhaps influenced
by this psycholinguistic processing--the student is marking the
pause in his/her own processing.
        --Bill Murdick