Because I'm too shy to join Sugarquill because everyone there is too smart and literary for me, here are some more thoughts from the Potterverse.

Lockhart must have a bigger, more important role in the upcoming books. Why? Every Defence Against the Dark Arts prof was very important in their own book and significant in the series as a whole, except for Lockhart. Quirrell introduced the threat of Voldemort. Lupin introduced us to Harry's father's generation, MWPP, and provided a vehicle for Sirius Black to be trusted and therefore for Harry to be hanging around the headquarter of the Order of the Phoenix. Crouch!Moody not only orchestrated the whole plot of GOF, but also enabled Harry to witness and be involved in the resurrection of Voldemort, and generally led to the dark tone of the series. Umbridge represented and enabled the Ministry's coverup of Voldemort's return and the resulting suppression of Harry, Dumbledore, the Order, et al.

But Lockhart didn't do anything. He was egotistical and inept, and unhelpful at appropriate times, but the plot of COS could easily have been carried out without him. And the fact that he had a cameo in OOTP shows that he was more than just comic relief. Besides, I refuse to believe that he was intended as comic relief because Wacky Character is the worst kind of comic relief and Ms. Rowling was able to include humour in all the other books without a Wacky Character. So what is Lockhart's Greater Purpose?