BISA is worth having if your entire family remained in SA, and more useful for a genealogist to cross reference other records.
My resources do not show an Arthur ROBERTSON in the Yankalilla registration district in the 1860s though there was a Robert ROBERTSON having children and living in the same place Tongulilla/Tonqulilla where Ruth was born.
I did notice a Marian MITCHELL nee TAYLOR having children in the same era in the same registration district though I cannot see how the Registrar would confuse the two as the MITCHELLs lived in different towns. Otherwise, TAYLOR may have been Marian's mother's maiden name and Marian was born and registered before the mother's marriage to Angus McLEOD.
Just some thoughts!