The MB-D100 grip is an essential accessory to the D100, offering:

• vertical shutter release, and shutter speed & aperture controls,
• better handling of the camera by improving the stability and balance,
• the camera can now use 2 batteries, for even longer life,
• accesory AA battery holder for emergencies,
• audio recording.

The Voice Memo Recording Function might not seem immediately useful, but I wish I had something like that when I was still doing socials & functions photos for magazines. Having to write down names of people (from left to right), for every shot, was a pain. It would’ve been so much more convenient to just have been able to record the names accompanying each photograph.

The accessory tray allows you to use 6 AA batteries to power the D100, and seems like it might be useful in an emergency .. but Alkaline AA batteries give you a very limited life-span of around 30 JPGs before being drained to the point where the camera becomes sluggish. But it may just be the thing to help you getting those last 20 or 30 shots if your main batteries become drained. With NiMH rechargable batteries you will of course get a much longer lifespan out of this accessory, and there are reports of 200+ NEF files shot on a single set of NiMH batteries. Of course, as in all things, your mileage may vary.

There is a gripe that older versions of the MB-100 grip don’t fit snugly, even when tightly screwed in. This is due to the ridge surrounding the mounting screw. This can easily be modified by sanding down the ridge as a number of people, myself included, have done. For details of what to do :
MB-D100 battery grip – not so snug