The reason it's bad to do it for several generations, is because there is a lack of genetic variety, and genetic diseases eventually begin to appear, such as hemophilia, deformities, etc.
The British Royalty was an example of this, several hundred years ago!
While linebreeding is less likely to cause problems in the first generation than does inbreeding, over time, linebreeding can reduce the genetic diversity of a population and cause problems related to a too-small gene pool that may include an increased prevalence of genetic disorders and inbreeding depression
