Adding an unnecessary "and" is better?
Which example are you referring to?
The "and" in the "I like ice cream, milk, cheese and yoghurt" is hardly unnecessary. I assume that you do not mean this example.
Methinks, therefore, you mean my suggestion for re-writing the example that deals with Agnes Warwick's will.
The two versions that were originally suggested were:
(1) "According to her will, when Agnes Warwick passed away, she left the bulk of her estate to Thomas Warwick, Alice Johnson, Penelope Smythe and Frederick Warwick."
(2) "According to her will, when Agnes Warwick passed away, she left the bulk of her estate to Thomas Warwick, Alice Johnson, Penelope Smythe, and Frederick Warwick."
Wendy had suggested that the first version implies that Penelope and Frederick would share one-third of the (bulk of the) estate. My point was that this seems a rather weak implication. Had a lawyer - Oxford trained or not - wanted to be clear in saying so (and most lawyers I know do want to be as clear as possible, sometimes going to extremes in doing so), s/he would have written,
(3) "According to her will, when Agnes Warwick passed away, she left the bulk of her estate to Thomas Warwick, Alice Johnson, and Penelope Smythe and Frederick Warwick."
Thus, I added the "and" simply to make the reverse point that you seem to be making - the "and" isn't used to replace the Oxford comma but to clarify that the drafter had intended that the last two in the series were to be seen on par with one another.
(Had I written that sentence, I probably would have written that good ol' Agnes, bless her soul, had left a third of her estate to Tom, a third to Alice, and a third to be shared by Pen and Fred.)