Religion provides hope
One common apologetic is the argument that religion provides hope, or meaning, or the assurance of the love of God, and that taking away a person's religion would be cruel. Hope is only one of several psychological benefits that are claimed to be associated with religious belief.
Whether or not any given religion provides hope or meaning has nothing to do with whether its claims are true.
While previous small studies have had mixed findings, a large cross-cultural study of 8000 people in seven countries found a statistically significant connection between religious belief and major depression. 
Is this desirable?
This argument is comparable to the statement, "It would be cruel to take away a coke addict's supply, since that person would succumb to withdrawal pains." While this action would likely cause temporary pain to the addict, in the long run, the former addict would benefit greatly from eliminating his dependence. Similarly, it is better to be able to find hope and meaning without having to resort to religion.
Religion also provides fear
This argument also ignores the fact that for many people, religion brings feelings of unhappiness. Guilt is one, for example. In many religions people are told that having sexual thoughts is a sin, and for younger people with average sex drives, avoiding sexual thoughts is likely to be difficult or impossible. A wide range of sexual actions, even feelings outside marriage or even within marriage are considered sinful. Furthermore, homosexuals are often told that they are vile and wicked. Some denominations preach that all humans, no matter how much good they do, are miserable sinners who deserve to be tortured for all eternity.
The hope that such religions provide is not the uplifting hope of improving one's future, but rather the hope of evading "just" punishment.
Many forms of Christianity teach that the majority of mankind will go to Hell. The fear of this may cause great distress. If one were on death's door and raised to believe that s/he may be going to hell, that belief could cause great stress. It could also use up energy that could have been better spent, possibly saying goodbye to loved ones or even fighting whatever life-threatening condition is affecting her/him. It could cause friends and relatives stress to think that perhaps a loved one that died is in hell. It is difficult to estimate how much stress and unhappiness is caused by the fear of hell and the suffering that one is supposed to endure while in hell. Many people may be reluctant to admit how much they fear that they are sinners and could be destined to go to hell. The fact that, in their minds, hell is forever, is very hard to even imagine.
Religion also causes fear of supernatural horrors in this life. Many people believe that witches, demons, or evil spirits are sabotaging their efforts in life or plotting to attack them. Normally, one would try to get a person to disbelieve in these sorts of paranoid delusions. However, if they are a member of a religion that accepts or even encourages these sorts of beliefs, it may be very difficult to convince them to let go.
Religion can also cause fear of non-believers or believers in different religions, because it demands loyalty to a particular ideology, which in turn would cause a believer to defend their ideology against all conceivable threats. This fear often leads to needless prejudice and sometimes violence. Even "moderate" religionists are often afraid of the decline of their own faith, even if they can't relate this decline to any definite, tangible harm to themselves or others.