It's like chrome wire spoke wheels on a Ford a Escort.

Unnecessary
(Some) Chics Dig It [CDI]
Harder to clean
May help reduce upward flip of muzzle on firing one handed at arm's length
Definitely a NO-NO for firing in "retention" position in self defense, unusual positions, or close to your face.
Shooters to the right and left of you at an open range will hate you as gas and noise blows toward them.

The S&W Performance Center does some pretty nice work upgrading their stock guns. Unfortunately, the Shield package does little to improve an already good mass produced gun. If there are parts of the stock gun you want to upgrade, you can do so easily. I don't recall reading on this forum of any group of owners who did/or desired this set of modifications. S&W just seems to have pulled it out of their armpit. However, it is very reasonably priced for some quirky upgrades.