It all depends...The first point should be made, that is is a MYTH that any piece of meat will only "take smoke" for xxx amount of hours...3, 4, 6 - whatever. I believe what you are thinking of, is the smoke ring. Once the internal temp reaches 140° +, then the smoke ring will quit forming. However, a piece of meat will still continue to take on the FLAVOR of more smoke - as long as you are introducing smoke to the cooking chamber.
Now as far as your question, if I'm cooking for my immediate family, I see NOTHING wrong with smoking a pork butt for at least 4 hours, and then finishing it off in the oven or maybe the crockpot for a few hours. Let's face it, if we all had 8 hours of free time every Saturday - we'd all be slackers
OF course, if you are a propane or electric smoker, I guess you can set it and forget it, but that's a whole other argument that I will chose never to get into ;-)
It is best (and true to the art of BBQ) if you do the whole 8 hours or whatever it takes, on the smoker. It is ALWAYS going to taste better. Also, if I'm cooking for extended family, friends, co-workers, etc...I'm never finishing off my meat in the crockpot - because I want to show I can do an authentic Que! For just me and the wife, however, I'll do whatever I have time for...I'd rather tend my fire for 4 hours and then finish in the oven, than not get to tend the fire at all because I don't have 9 hours of free time!