There are two platforms here (IMHO). One is the competition level or platform and the other is just good olé' home cook'n. With ribs or shoulders there are things you might do for competition that you wouldn't necessarily bother doing for good ole' home smoking or BBQ'ing.
Example with Ribs - in competition you trim off the loin chunk of meat on the rib for consistency purposes. I would never do that for home cooking, it is a wonderful part of the meat.
Example with shoulders (butts, picnic or shoulder) - Injecting with what ever recipe you want. It's a lot of hassle, cost and time for a home shoulder. You can make a wonderful juicy, tasting, smoked, pull-able butt, picnic or shoulder with out injecting.
Now, with all that said; let's discuss some reasoning behind using an injection in the first place. The acidic traits of apple juice, etc, vinegar, certain types of wine assist in breaking down the collagen in the shoulder; introduce moisture to the meat, and add flavor.
The best injections from my experience have been the simple ones that use a single primary vehicle (the juice, wine, etc) for transporting the seasoning and breaking down the collagen, one or two sweetening ingredients such as molasses, corn syrup, maple syrup, etc and your favorite dry rub (usually strained before injection).
These are opinions obviously. Experiment (that's 90% of the fun, right?) with different things that seem to strike your fancy. Don't just learn that a lot of competitors use apple juice and or apple cider as a base for injection, but why? Create to your taste buds and enjoy! Good luck!!