Thing about the Magpul forend is that it was designed to be held with that Chris Costa style of grip hold. A wrap around technique similar to an AR-15.
Like this:
http://i.ytimg.com/vi/DS9-LDT8clU/hqdefault.jpg
Not like this:
http://www.gunsandammo.com/wp-conte...ries-at-gunsite/mossberg_magpul_shotgun_g.jpg
So, when users take the traditional grip on the forend they find it lacks grip, causing many to add grip tap to the grooves as seen above.
I hate the look of that magpul stuff, id rather have the traditional look myself.