"She" probably came from some attempt at political correctness. There's been a rise in recent decades to use "she" when speaking about an unspecified person. Historically, it was common to use "he" for that task. Unfortunately english doesn't have a gender-neutral pronoun and until/if one becomes accepted this will probably continue to be a confusing issue...
I've never heard a sentence in actual usage[1] where the usage of the singular "they" was ambiguous.
In terms of being understandable and elegant, the singular "they" is probably the best English can do. Unfortunately, it's not widely accepted because it "sounds" uneducated, despite being the current best solution to the problem. Gender-neutral neologisms, like Spivak pronouns [http://en.wikipedia.org/wiki/Spivak_pronoun], are not commonly understood, while constructs like "he/she" or randomly alternating between generic "he" and generic "she" as in the OP is inelegant and confusing (hence this whole thread).
[1] I'm sure you could construct a contrived example.
Written language has more legitimate uses than completely formed sentences. Pronouns can be troublesome enough, figuring out the antecedent, without requiring yet more context analysis.
Yeah. I caught that too. So awkward. As a she, i'm all for some "she's" in abstract examples, but they switched pronouns in the middle of an expression. Strange.
Where did the she come from?