There are several advantages to mopping, depending on the cut. I don't necessarily open a pit to mop, but if I'm going to open the pit anyway, why not. NEVER put cold mop on hot meat. The cooling will definatly add time to your cook.
Mopping adds an extra layer of flavor to your bark. I mop a whole hog at the flip, and everytime I check the progress of the cook. Thermometers are a great tool but, tell whether a hog is done to standard takes visual and physical checking. The right ingredients will also change the hues of the meat, making your presentation better. Finney has it pretty much catagorized.
I also mop chicken, with a primarily oil based mop. In fact I've been known to spray straight canola on chicken. It makes the skin turn a really nice mahogany.
Ribs require something sweet like apple juice. There is nothing wrong with layering flavors onto a slab or ribs as long as all the flavors compliment each other.
I prefer spray bottles over brushes. Less of your rub goes away.
Good Q!
Jack