It depends. Terminal illness or permanent and severe psychosis or dementia, then I would say yes (with a living will to establish informed consent in cases where the person's present state would prevent them from giving informed consent). I would go as far as to say that if you cannot give informed consent to die you also cannot give informed consent to overrule the wishes you made clear in your living will when you were capable of informed consent. I wouldn't want to set out a living will saying "give me a pill overdose or injection if I get Alzheimer's" and then have it not happen just because my 'self' with Alzheimer's refuses to swallow the pills or is afraid of needles.
Of course absent a living will the presumption should be that the person would've wanted to live, but with a living will protestations given in a state where informed consent cannot be given should be ignored. Informed consent cuts both ways.
We can't know what is going on inside an individuals head but what if a permanently ill person is just dreaming of dying and it's the only thing that makes them happy, we all assume suicide is a dark thing for the melancholic people but maybe it's the only things that keep them going? Then again it may only be used as a last resort for people who and bare the pain and can't live in the body they have been acquainted with. Personally, i think thorough physiatric evaluations should be given to see if there really isn't any help, people think if someone is feeling down or depressed they can throw some anti depressants at them and tell them to talk about their feelings. We need to think in the long run, the world is full on unhappy people who use suicide out of hate but surely we could grant it to those who are dying?