An Master of Arts was an absolute requirement for my PhD, regardless of undergraduate grades. This is because many arts subjects are not available at undergraduate level and, (with the possible exception of arts practitioners who have stacked up a wealth of professional experience), an MA represents the only way a university can evaluate your suitability to go for a PhD in the subject. In other words, for many arts students the MA is an actual shift in subject rather than the next step up - making it not a matter of it being worth doing, but obligatory.