Human cloning is dangerous to mankind when viral or other harmful components can be transferred, along with cloned tissue, into new hosts. That is the first area of concern, as cloning does not just involve snipping and clipping at the sub molecular level. The tissue must be grown and harbored in environments where contamination can exist, no matter how stringent the controls are over hygiene and disease prevention.
Human cloning gets certain radical and extremist segments of society worked up, just on the basis of their religious interpretations of mankind's ability to "try to play God". Messing with perceived limitations on mankind's allowed endeavors that come from divine imperative can result in social disruption, international diplomatic disputes, and even wars.
Human cloning, where animal vivisection or mistreatment is involved can get other extremist elements of society worked up enough to engage in sabotage, violence, and actions against those who do the work, their facilities, and any innocents who may be in range.
Human cloning is dangerous to mankind when our evolutionary tweakings are not well understood. Yes, it would be good to eliminate Sickle Cell disease, but the Sickle Cell trait offers some protections against Malarial parasitic activity, which is one of the biggest killers of humans. There are reasons for the mutations which have occurred in the Human genome, and if they are not completely understood, even the cloning of tissue can result in mistakes that we just do not understand.
If human cloning ever gets to the stage where a completely functioning human can do "survival of the fittest" (which, in the truest sense means surviving for long enough to reproduce and to bring offspring to adulthood), no one knows which new disorders and mutations might be passed along, or whether those disorders and mutations will be new, worse, or devastating, when passed on to generations of offspring.
Human cloning tests the limits of scientific method against the limits of mankind's acceptance of science at all. The potential for scientists to do something that is so horrific that whole societies create a backlash against scientists, is getting higher and higher. Just as literature has explored the imaginary potential of scientific accomplishments, literature has also explored the ways in which scientists can be made to pay for excessive attention to results at the expense of that which the supporting and surrounding society holds dear.
Human cloning offers great hope in eradicating disease, healing the sick, and making life better. But there is a line that should not be crossed, and what is given to scientists by society, can be taken away if the human trust in them is violated in arrogant, egregious fashion and without sanity, rationality or remorse.