Steve/Steve--
Adding moisture and flavor to turkey that's going to be fried definitely has merit, imo. When I fry I'm not rubbing so I inject for both reasons. When I smoke turkey I brine. If I want the rub to be more substantial I use a straight or mildly flavored brine. Often I use a more potent brine and a light rub (or no rub).
The dynamics of the change in turkey meat during cooking is different from that of pork butt. Butt doesn't need the moisture or fat addition of an injection--my opinion on the flavor addition I noted above. I am of a similar mind with brisket except that brisket can often use the moisture/fat addition (especially many of the ones I buy). For a brisket injection I'm more inclined to one of fat plus beef jus or stock (with its subtler flavors), with minor additions of some of the rub flavors (which works, imo, because brisket is sliced and not pulled). I still want the rub/bark to do its thing and my focus, like with butt, is more on that. For pork I'm going to slice--particularly fresh ham or loin--I'll flavor-brine or inject and base the rub on what I expect the result to be once cooked. But I'm looking for a different sort of result in this case, I cook at a much higher temp, and the rubs I make are thus quite different than what I make for 'typical' low/slow barbecued pulled pork.