May I ask why you feel you need one? I just hate battery grips. They take up space in my camera bag, they add weight to the camera, and the vertical shutter button is not as comfortable as I thought it would be.
That's just your opinion and experience - if ours were all the same, we'd be married to the same person.
Adding weight isn't always a bad thing. I have a battery grip for my 550D and the added weight of the battery grip (and batteries) balances the front-heavy feeling when a heavy lens is attached. Taking portraits feels more comfortable for me with the grip as well.