These little items, much hated by pedants (or should that be pedant's?), are commonly thought to be a symptom of declining educational standards. Readers may therefore be interested to know that on the Antiques Roadshow of Sept 16th, one viewer brought in a catalogue of "Historical Curiosity's" from Alnwick Castle. It was dated 1770 and written by a well-bred - and presumably fairly well-educated - young woman.

So, Potatoe's, Cabbage's and Plum's have a respectable historical precedent.

[Edited] - a good one I saw today using plurals in a creative way: "Partie's catered for"

Greengrocer's apostrophe: the lowdown from Wikipedia