I think people who sincerely believe in religion and spirituality do have some problems they'd be better off without. They are essentially investing their confidence in things that won't help them. They're counting on things that will never come, and possibly even going so far as to diminish their investments in real, practical answers so that they can further invest them into useless things. Granted, religious and spiritual people do sometimes seem to have a benefit from the positivity of their beliefs, but that's basically saying they're gaining bliss from ignorance, and that's an approach I've never trusted. I tend to think that eventually your ignorance will open you up to a problem severe enough to shatter all of that and leave you worse off than if you were never ignorant. Some are lucky enough to not have to face that.
On the other hand, even the whole ignorant bliss thing doesn't always pan out. I've heard from so many people who struggle with their lives, and they ask why God has forsaken them, and it's really a horrible thing to hear. There are people who live in fear of hell. There are people who take their losses as some kind of punishment for inadequacy. A statement that really stands out to me in this regard is that of a chronically sick woman who asked "If I can't believe enough to be healthy, how can I believe enough for salvation?" Think about that.
This might be straying a bit outside of your question, but I definitely think that it's better for other people when person isn't religious or spiritual, and in the big picture all of us (including them) would benefit as a result of everyone shedding such beliefs.
That being said, I rarely try to change the minds of religious people. In most cases that it has happened, the issue was pushed by the religious person and not myself. The reason for this is that I simply doubt my ability to change their minds, and the process of trying tends to be turbulent. I think the conflict often ends up being costly for all concerned, but since they rarely ever change their minds, nothing is gained for that cost.
The amount I care about someone makes me more likely to try. Obviously, since I think it's better to not be religious or spiritual, it's partly compelled by my desire to want something better for someone I care about. There are other factors, though. I try to maintain relationships with people I care about, and the more time I spend with someone or the more personal the level of interaction becomes, the more problematic our different philosophies become, so it just follows that attempts to change their mind will be more likely.