I beg to differ. The ability to think is an essential byproduct, but knowing Shakespeare is inherently important.
However their ability to think is still limited to merely the thought processes of an author for children ( Disclaimer: I have no intention of belittling Enid Blyton here).
By just reading Enid Blyton you can't think in terms of metaphors or similes or puns etc. By reading Shakespeare you gain an inherent ability to think in a completely new way. This is also perhaps very similar to what PG says in ANSI Common LISP about LISP's unusual appearance, to quote PG: ' You have to think in a language to write programs in it, and it's hard to want something you can't describe. When I first started writing programs-- in Basic-- I didn't miss recursion, because I didn't know there was such a thing.' Similarly if you didn't know there was such a thing as personification, you wouldn't miss it!
And having some common points, like all studying Shakespeare, is good because it means we can all talk and think about it together.
Studying the humanities is not necessary for a person to learn to think. I didn't mean to imply that. But studying the humanities can certainly help in that direction, and should be done anyway.
I understand why people hold it up as essential: it's how they learned to think, and they can't think of another way to learn to think. So they call it "essential." It's still wrong. But understandable. Learning to think comes from having diversity in experience, not reading Shakespeare.