I inject any pork I cook, butts, shoulder, whole hog, etc. Pork is basically bland and the flavor from the juice and other seasonings really enhance the experience. My basic injection is apple juice with just enough sugar and salt so that you can taste it -- in other words, a very light brine. You can add rub or other flavors as long as you strain it so it doesn't clog the injector.
Now, I do have a brine injector and that makes a huge difference in the amount of labor to get a whole hog injected.
I've eaten hog without the injection and it's good, but the additional moisture and flavor is noticable.