You seems to be convinced that there is a logical answer to this question, but we all know that that is not always the case, when dealign with the government !
Back in the old days, only joint assessment was possible, and when a woman got married, her PPN was taken off her, and she was issued with a new one, which was her husbands number with W at the end. Also, her tax free allowances were taken off her and given to her husband automatically.
Then women, to the shock and horror of all, decided to demand to be treated like full members of society with rights and things, and demanded the right to be assessed for tax in their own right, even though they were married. Many thought that this would mean the downfall of Irish society but amazingly, we survived, and now couples have a choice of how to be assessed for tax.