Ahh, following in the steps of the S&W M&P
At least on the M&P, the ambidextrous manual safety is there if you want it but you don't have to have it on or not -- the other safeties will take care of you regardless. It's an option, just as ordering yours with the magazine safety or the internal lock would be. All up to your preference.
I will say that the one they're showing on the XD's page is stupidly shaped almost exactly like the slide release and right next to it. Very bad idea in a high stress situation unless you really are simply using it as a safety switch and turn it off *before* the shooting starts... But most people will use it as a thumb operated safety. Glock's coming out with some sort of manual safety as well which also has a really bad location/shape right between the similiarly shaped and textured magazine and slide release. And it appears you could way too easily push it too far out the other side of the pistol and it's on safe again. :? - Schofield