Boy, Keith, I couldn't disagree with you more.
The birth fact is just that - the birth fact. It has nothing to do with various relationships that person may have with another person - in particular their parents. The birth info from a census, for example, is just a calc from an age and a place - that's all.
You say that proper sources of a birth indicate the person's parents?? You must operate in a different world than I do. I recently started looking at Canadian records and if that is where your research lies, that is true - but only for birth, marriage and death records. This is certainly not true for US records. The parents are shown (normally) when age is given in the WWI Reg Cards, any military record, many census records - at least as an adult, and sometimes as a child - nor many US death records or even US marriage records.
As far as the KIS principle - documenting the relationships that a person has, particularly their children and parents, but others, as well is the MOST important of ALL the attributes of what we do as genealogists.
There is only a need to go to these Custom Facts because FTM does not provide any structure for documenting relationships, only facts.