Cloning is the production of a living replica of a life-form. This is usually accomplished by inserting an organism's complete chromosome set into a newly fertilized egg of the same species, substituting one set of genes for another. (Source: R. Gerber, Vibrational Medicine, Glossary).
"Cloning is an asexual method for reproduction that uses an original seed or genetic stock for replication." (PoL)
To Earth-humans, the science of cloning is very young. We do not have a lot of experience in the field. There is a theory that states that clones, like photocopies, lose quality compared to the original.
Some of the abductions by aliens are claimed to have taken place in order to get genetic samples for cloning. The reason for that would indeed be that because over generations of cloning, the stock to clone from has lost too much of its original quality.