Depends. Mustard or oil depends on the flavor profile I want to end up with. If I am using just a commercial rub , mustard is used. If it is a one by one rub build then I use oil.
Everything I put on meat will add to the flavor profile, even the glue. I don't use anything that doesn't translate into flavor. For ribs, chicken, brisket, tritips etc I use worcestershire sauce (also adds umami, google it if you don't know what umami is) and Annie's Roasted Garlic EVOO as glue. On butts I prefer dark molasses as glue.
I used some Molasses last time (was doing a bunch of butts so I had the bottle out). Worked pretty good, but adds a sweet/earthly flavor that might be a bit on the strong side.. Otherwise I use Carolina Treet.
For me it depends on whether I want a heavy or a light coating of rub. If I want a heavier coating of rub I usually use CYM (Cheap Yellow Mustard). For a light coating, I just apply a light coating of rub after the ribs have been washed off.
Some people swear by vegetable spray, but I just don't care for the taste.
From time to time I'll use cider or malt vinegar (CYM & Worcestershire also contain vinegar). It adds flavor and supposedly helps open the pores of the meat allowing deeper penetration of the rub & smoke.
CYM seems to work well on ribs, I also use it as glue for my steak rubs. I find it helps tenderizing the meat as well as adding a nice flavour profile.