It causes a lot of mess/trouble/etc.
The user should be awared that any new conversion in date formats should cause a renewed conversion of the relevant fields allover the (entire) GNO file. In spite of this, presenting the Christian date and the non-Christian one in parallel, makes "defaults changing more than once under same user" mostly unlikely.
In almost most cases, I've forgotten to change some isolated date somewhere, and this caused the death date to become lower than the birth one, a.s.o.