The BCL is a very very prestigious degree although it is somewhat suited more for those who wants to branch into academia. An LLM or BCL of that prestige would give you a leg up if you are fresh out of law school gunning for the big firms and your LLB was from a "lesser" uni. Otherwise, there isn't much reason to pursue it other than pure interest in the study of law. Lets just face it: Law is all about prestige and names....