They put your hand in the same position every time, and help keep it from moving under recoil for two shot games. Anyone using an EVO-COMP stock has one, and a lot of wood stock shooters have them as well (ISSF shooters).
I think they are advantageous IF the fit the shooters hand properly. I wouldn't mind trying one if I were having a custom stock made, but I'm not sure I'd buy a pre-made stock that way.
Worst case scenario if you don't like it, just grind it off and you have a traditional grip.
I've thought about making my PFS grip into a "glove grip" but it might make opening the gun a pain since I'm a lefty and have to PUSH rather than PULL the top lever to open it, and the "horn" might get in the way.
I'm a lefty like Ian and tried a Perazzi left hand version with adjustable comb, it felt great and seemed to give more control. I wasn't ready to bust out $1,500. May have done so If didn't have three PFS.
I had to loosen my grip reach for the lever. I've had several Perazzi's all except one that had a large radius grip and the glove fit I could open without letting up on my grip, I do however have small hands. The short answer, it wasn't that big of deal,