From a spiritual perspecitve, we're basically keeping the soul from passing on because we're not letting the body die. It could be a kind of hell being trapped in a body in that way. If whatever god or gods indeeded the person to live, they wouldn't be in that state to begin with. They would already be dead if it weren't for modern medicine. Might as well let them meet their maker or gods.
Looking at it from a more logical or objective perspective, if a person is brain dead, they're more or less dead. It's just a waste of resources to keep them alive for the vain hope that they'll come out of it.
It's really a selfish decision to keep them alive in my opinion but if a family wants to keep the person alive with their money then by all means. Personally, if I end up in a coma with little chance of recovery, let me die.
