Here are my views on the bonuses of grips (which I use on all my cameras)
1) Pinkyfinger - as just mentioned, the grip provides a point on the camera for your right hand pinky finger. I don't have big hands at all, and yet even I find that the pinky finger often falls off the bottom of an ungripped DSLR. The result is that it rubs on the corner, which is an annoyance. The grip gives you a spot for that finger - making holding more secure and easy.
2) Wrist strap attachment option = yes there are some that don't need the grip, but these tend to get in the way of tripod mounts and at not quite the same. With a wrist strap (eg canon E1) you can have an easier time holding the camera. This is especailly the case when holding it in an -at rest - position (eg dangling down) as you can relax your fingers and rest your grip against the strap a little. Wrist straps are also fairly small so you can easily throw your hand over it and shoot fast if needed.
3) Portrait aspect controls. Shooting in portrait is a pain on your wrist and provides a very weak hold on the camera if you've only got the regular controls. Furthermore shooting for a prolonged period like this is even more a pain. Everyone uses portrait mode and the grip gives you the controls you need to easily and effortlessly shoot in portrait just as easily as in landscape.
4) Increased battery life. If you use two at the same time you get increased performance (as they support each other when running lower). That said I always make sure I carry a spare battery outside of the camera, even if that means I only have one in the grip. That gives you a safety net - should the battery in the camera die you've got a spare to flick to and a mental note to recharge the other. Rolling with only the batteries in the grip and you run the risk of running out of power with no backup.
5) AA Battery use - ok I don't use this oft (ever) myself, but if you're out in the sticks with nothing else you can use AA batteries if you take the carriage for them with you. That said chances are you won't get many shots and further if you're heading anywhere where you can buy AA batteries chances are they've got a power socket.
Warnings with grips
1) Don't over-tighten the screw - pretty much all grips have a cog system to screw their locking screw into the camera. The downside is that most of them are pretty weak setups - they can easily lock in very tightly - but then the grip is likely to get stuck as the cogs are not strong enough to counter once the screw is locked. So never overtighten the screw - if you do you'll get your grip stuck on the camera (my 400D has this - personally with a wrist strap I've never worried as I don't need to take the grip off)
2) Battery power levels with IS lenses (I assume the same for OS and VR) can mean that you get a sudden power loss with grips. I tend to experience this as the camera working fine, then as you take a photo it dies (mirror locked up). I assume the camera budgets for the power for the shot, but the running IS (and maybe even AF systems too) sucks out the power needed so that it simply runs out.
However it happens if your camera suddenly locks up the fix is simple. Turn it off and take out the batteries - replace with fresh ones and turn it on again (not you must turn it off first as with no power it won't actually go through its off cycle, so the mirror remains raised). This is generally not a fault (the batteries just ran out of charge) but it can catch you unawares the first time it happens)
3) Some grips are not as weather sealed as the camera body you attach them to (oft case for some 3rd party options). So do check the weathersealing status of the grip if this is important to you in extreme conditions (though remember that most extreme conditions will call for additional layered protection anyway).
In general I consider grips very important - at least very helpful with photography. They give you a lot of additional creative options with easy portrait shooting - whilst also giving you bonuses to battery life and support.