I've tried both ways with and without couldn't really notice a difference in the ribs. I usually marinate now and have been brinning the ribs I cook for a short time with a light brine before. If you wanted to try sometime, I don't know why it is but instead of using olive oil, soybean oil seems to do something for the ribs. If you like going the mustard route, the addition some fresh lemon gives it a little bite and brightens up some of the heavier flavors of rubs.
I started using the yellow mustard some time ago, and I do think it helps hold the rub on more evenly. I've never noticed a mustard-y taste though. All in all it probably doesn't make that big of a difference, but I'll keep doing it anyway.
I usually prep my ribs and butts the night before a smoke and just apply the rub directly on the meat. While the meat sits wrapped in plastic wrap in the fridge, the salt in the rub will pull moisture from the meat and turn the rub into a paste. The day of the smoke, I sprinkle on some more rub and then the meat goes into the smoker.
On the rare occasion the the smoke is a last minute thing, I'll rub the meat with yellow mustard and then apply the rub.