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.