How does genetic variation make a community more stable?

Normally, members of an average community have downfalls in their DNA. However, as each being is completely different, different imperfections affect different people. 

Now, imagine a community based on the offspring of two people. The imperfections passed on through the family line will grow stronger and stronger until a disease rare to normal human life strikes the family.

This is one of the reasons why incest is illegal (although still pretty gross even without that fact.)