I use a simple rub of garlic powder, cracked black pepper, Kosher salt, and brown sugar on ribs and butts. What I typically taste after a cook is the sweetness of the brown sugar. I can detect some flavor from the garlic. Having smoked with wood/charcoal and electric, I will say that rubs do help in producing a decent bark. I have smoked without rubs and the bark is different, very dry but tasty in its own right. With that said, maybe a rub helps retain moisture. I have truly never paid that much attention. Since I make my own, it is inexpensive and just part of the ritual. I will say that the brown sugar sort of liquifies and during smoking runs off the meat and probably takes most of the other stuff with it.
You might try letting your rub rest on the meat for several hours/overnight in the refrigerator prior to smoking. Another method of introducing flavor is by injecting liquid flavoring/marinade deep into the meat. Just experiment.
Use the search bar at the top of the page for rubs. Also, the Smokin-it site will be of help.