My extensive experience with brining is that it adds flavor to the meat through and through, plus it denatures the protein (breaks down the structure), changing the texture of the meat giving it a smoother, uniform bite. I actually prefer to brine turkeys, chickens, and pork because the meat is more forgiving of cooking errors and always comes out moist, delicious, and pleasing to the tongue. I don't brine beef unless I'm going to make corned beef, then you need a special pink salt. Marinating beef though can have the same results as brining.
To answer your question directly, if the shoulder is brined for 18-24 hours with the flavor profile you desire I can't see any advantage at all to injecting unless you were trying to add a different flavor to the profile. Skip the injection of a brined shoulder and use rubs instead to finish the flavor profile desired.
As others have said, you don't need to brine or inject, so try it all three ways and see which you and your guests prefer.