all i can say is i have seen a BLM healing the last boss in stone vigil more than once (even in full 50+ gear without any mind on it) after the healer has killed himself. if the BLM would have attacked instead of healed it would have been a wipe, because the boss had too much HP left to burn him down.
there are also thousands of posts all over the forum about why a healer should dps - so why should a BLM not heal, when the actual healer is dead, when it can save the run?