First reason for many others to get Hermione to Harry! :)

Prof. Lupin always says that Hermione is the smartest witch of her age and she proves this to be true several times. More important than the genius of Hermione, is that it teaches the magic Harry really needed. Hermione teaches Harry Accio the spell before the second task of the Triwizard Tournament, she learns how to use the Time-Turner to save Sirius and she is literally the reason Harry survived in "Order of the Phoenix." Ron helps whenever possible, but does not add much wisdom of truth to Harry. On the other hand, Harry Hermione teaches about friendship through acts of loyalty and bravery, things she could not have achieved with Ron.