In the case of terminal illness, I think a person should choose whether to die or not.
In the case of brain dead, I am only concerned for legal ramifications in this issue. There needs to be a lot of barriers, double checking, triple checking to make sure no one takes advantage. Personally, I'd want someone to kill me if I was brain dead.
If someone is brain dead, they are dead to me. Sounds horrible I know, but that's life. Think about it this way, if someone you loved all of sudden is brain dead, are you telling me that you are still happy because they are still alive?
Also, if you are brain dead, you don't mind staying alive? Wasting bills? Burdening on others? Wasting food? Pooping on yourself? Peeing on yourself? Because of that 1 in a million chance you may wake up?