Stan nailed it for all of the good reasons of why to get one. To add to the discussion, years ago I contemplated the same question but I couldn't find much on whether the arguments of 'it feels better for those with big hands' held up for a woman photographer.
I am a small woman and worried that having small hands would make a grip unusable for me. I will tell you that the balance is better in your hands, small or large. I have never had a problem with handling it so I don't think the size of one's hands makes any difference. It's really just the balance being much better.
I got my first battery grip years ago on my D90. When I got my D7000, I didn't think twice about getting a grip for it. I use the AF-On for focusing and would be really constrained when shooting portrait-orientation if I didn't have a battery grip.
As with anything photographic, there's always a trade-off. Yes, it's heavier and that's a drag but....it's so much more functional and therefore, I put up with the additional weight.