I suppose that it could just be a sign of the changing times that we are living in, more's the pity.
Suppose a man marries, then divorces, and remarries again, then wife No 1 would have the same RSI No as wife no 2.
Where some of these foreign religions and cultures allow multiple marriages at the same time, the whole Irish system would collapse, if all the spouses of the male, kept his number with a W added on. At least if a person has the same number from cradle to grave, the State will always be able to keep track of them and their various relationships.