TBH I disagree. In some fields it has become something of a convention, especially when the thing you're pluralizing is a variable or some other mathematical expression. For example I might write $x_1,x_2,...,x_n$, and then refer to the sequence as "the $x$'s". Happens all the time in professional journals by authors who know very well what the prescriptive rules of written English are. It is just easier to read this way.

Its defiantly against the do's and donts of English, yes, apostrophe's seem to be the most mistaked parts of English. Also my favorite cigarette's are Marlboro 100's.