Do they share a common paternal line ancestor? They lived near each other, moved from place to place together. But are they descended from the same man? There are many links between our Kerr families living around the world today. Paper trails are not getting us there. Kerr family historians need more Kerr surnamed men to have their YDNA tested to identify the missing links. A DNA match between two families establishes a high probability of common paternal or maternal line ancestry. Knowing two families may share a common paternal line can identify where more concentrated paper research may identify that common ancestry. Given the state of paper records, the DNA result may sometimes be the only information indicating common ancestry between families. Several people I know who are in that situation are pleased to at least have that information. Situations where there is the sole living male of a paternal line and no likelihood for male progeny really need to be tested if family historians are to have this information to aid their family history search now and in the future. In many cases, family historians will have to muster resources and pay for tests of individuals to obtain the DNA signature from people who have no personal interest in the result. All DNA projects accept donations to allow testing by those who need the assistance, so everyone can help out by making donations to DNA surname prjects at the major DNA test companies.
Any serious genealogist should want all of the maternal and paternal lines tested now that we have this tool.
So, join your fellow Kerr Family Historians and do a DNA test, you will spend the relatively small amount of money a DNA test costs on far more frivolous things over the next year, but this one will last forever, so do it now, you'll most likely will never miss the money, but you will treasure the result. For a family historian, DNA testing should be right up there with checking old paper records.