Suppose I'm playing a multiclass fighter/druid (because that's how I Range). I've got both Wisdom and Dexterity above 13, but Strength's only 10.
I annoy an efreet, who uses wishes to reduce both my Wisdom and Dexterity to 12. (Because my GM's a lawyer and knows that I care about the rules, and that this will tie me into knots.)
It's time to level up, and I don't meet the prerequisite for either of my classes. Probably don't meet the prereq for any third class, since I was so focused on Wisdom and Dexterity! Am I stuck until I otherwise raise a stat?