I have the Mark II and a ~9cm-wide right hand (with what looks like a 21cm circumference) and the built-in grip is very comfortable for me. I don’t feel the need for an additional grip. (But then I haven’t tried one, so I may not know what I’m missing out on.)
The advantage of the Olympus grip is that it not only gives you a place for your little finger, but it protrudes forward somewhat at the bottom below your finger to give a very secure grip and it also holds a second battery.
Actually, I almost forgot but I do have the HLD-9 grip. (It was given away by Olympus when I bought the camera, and now it sits unused in my drawer.) If anything, I find that it gives me perhaps a somewhat *less* secure grip by interfering with the ability of my left hand to support the lens. (I find it hard to give a definite answer but the additional bulk means that it’s almost surely a net negative for me overall.)
I don’t really feel like my right pinky needs much help anyway—the way that I hold the camera, most of the weight in my right hand is on my thumb and my middle finger.