Knowledge is useful and can be made to benefit mankind. Incorrect knowledge is a detriment to the collective good. Religion is an idea about a false concept of reality that leads to many wasted lives. To make decisions it is arguable that we must have emotions (http://en.wikipedia.org/wiki/Emotions_in_decision-making) and rational thought alone is not enough to impel us to choose. Religion, alongside other disagreaable memes, ride upon emotion and hijack the decision making process.
Religion distorts reality so that rational thinking is impaired and the net result is immorality and mis-aligned evaluations of worth or 'rightness'. Religion takes an idea (for example procreation) and turns it into an unnatural formula (only x or y constitutes acceptable procreation , only p or q relationships are allowed etc.) which is socially sub optimal and can only be maintained via immoral behaviour (reject r and z style relationships via social stigma or physical force.)
Without religion we would have fewer pyramids (and the misery they took to build) and more moon bases. Without religion we would have conquered almost all illnesses. Without religion we could tell our children the truth.