Mike,
Ribs - I've gone to wrapping ribs to obtain a very consistent result, mostly for competition purposes. When to wrap, that's sort of a judgement call. I look mostly for color of the ribs and some pull back of the meat from the bone. The next judgement is how long to leave them in foil. Typically its 45-60 minutes. BTW I cook mostly loins.
Brisket - I've only recently begun breaking the foil out here. In trying to maintain juiciness, I might foil when two things occur, 1) color is where I want it, and 2) the meat has come out of the first plateau. Its not really a prescribed time, it just a feel for when to wrap the thing. Results have been promising. Having said that, I'm looking forward to a cook sometime this fall after competitions are over where I'll just coat w/ salt and pepper and let it rip.
pork butt - in all honesty, I don't have all that much experience wrapping when cooking on the WSM. I'm not sure I would. I'm moved to cooking comps with an FE, and part of the process I've adopted there is to use foil. Again looking for color and sufficient completions and some textural signatures.
Some folks call it a crutch, others a tool. I can say for me, I'm able to achieve more consistent results with foil, but when I'm cooking to feed a bunch of folks, forget it, its not worth the effort or the money.