Some states have begun to require a "loaded chamber indicator" in order for a handgun to be sold in the state. I believe CA is one of these, MA may be another, and there may be still others. If a gun is designed with one from the start (the P-38 comes immediately to mind as an early example) then dandy. But if you haven't designed that feature in, adding one can be a real engineering challenge.
In the case of the M&P, adding the little hole at the back of the chamber will satisfy the bureaucrats and let them keep selling their wares in California (or wherever) without having to go back to the drawing boards and redesign the whole slide, then change the tooling to manufacture the new design. Whether it does any good or not is largely irrelevant, because it is not a feature that the market is demanding - so doing the minimum is all that a prudent businessman really should do.
Note, though, that if loaded chamber indicators became a feature that gun buyers really wanted, designers would design that feature in from the beginning, would expend some effort to make their indicator better than the competition's, and would spend advertising dollars to trumpet their "better" feature. But in the absence of such consumer demand, there's normally no business reason to add complexity and cost to an already complex and costly product.
