Rub sticks fine without anything. Mustard just makes a gooey mess. Save it for your hot dogs.

I use mustard, and it does not make "a gooey mess". Is it necessary ? Probably not , but have always done it and had fantastic results.

I have had good success with mustard but is it a requirement-nope.  I use worcestershire as a binder for rub on beef ribs.  The much greater flavor contributor is the rub you use.  FWIW-

I sometimes use oil for ribs because it seems to make them a bit more tender. I suppose its because the oil traps some moisture, sort of like a foil wrap, but not so complete.

If I have it on hand, I use a kind of oriental peanut oil that tastes and smells as if the peanuts were roasted before the oil was pressed. Also, sometimes mixed w. some toasted sesame seed oil.

I suspect that some of the mustard tradition is because along w. the rub ingredients, it helps sterilize the surface of the meat.

I use mustard and I learned it when I took a cooking class for brisket and pork butts.  Plain yellow mustard isn't really going to change the flavor much.  It's used a binder for the rub and helps with making a good bark on the meat I was told and have have found when using it.  I use it on brisket, pork butt and ribs.

Olive oil is neutral and doesn't add any flavor and I have used it when I cook a whole beef tenderloin to help bind the rub or even on my steak.  Another thing to use when grilling direct and doesn't flame up is grape seed oil.  It also helps keep the food from sticking to the grill.  I guess a good test would be to do one pork butt with mustard and one without to see if it makes a big difference.  I have been doing it since I started and it comes out great every time so why mess with a good thing if it works.

  

Long ago I read that there is no point in letting rub sit for hours on something that is going to cook for hours anyway.  There is plenty of time to absorb that flavor on the grill.   So, I usually rub and go!  Less effort, results are great.

That said, I like the suggestion to rub and wrap, for the purpose of making a wet rubby goo that will hold more rub.  I may try that some time, but usually don't plan that far ahead.

I have to tell you, I don't think any of it makes that much difference when you're doing a low and slow.  Some guys here use nothing but S&P.  Always wanted to try that, but never want to mess with what works to experiment...

Do you guys that use only S&P on your pork and brisket still get a nice bark?

For me the key to a great bark is the pepper.  I go heavy on the pepper compared to the salt and add paprika and garlic.  Never measure just use the taste test.  FWIW-

I use mustard on almost everything that gets a rub or spice blend, even the prime rib Christmas roast. The mustard serves as a thick paste that holds the rub in place.  Works very well, makes an excellent crust. No real flavor other than bringing out the flavor of your meat into the crust. So easy and simple, I really don't understand the "unnecessary extra step" comment. I can definitely tell from the result when mustard  is used and when it is not. My family and I like the result when I use the mustard crust technique. adds body the density to the crust. 

I use mustard but not every time.  Sometimes on a butt I use peach preserves.  I think both allow more rub to adhere to the meat thus creating more bark.  Also rub 24hrs in advance if I plan well enough (which isn't as often as it should be).

+1 for simply dry rubbing the day prior and fridging it til the cooktime.  It's already messy enough as it is without more goop...  And I put 'em on cold, out of the fridge - love that smoke ring!

i also look to add a bit more brown sugar to the rub for pork shoulders - not for the sweetness, but for that goodness which is the BARK!  My BIL and FIL were practically fighting all weekend long for the largest bark nuggets they could find in the 19#'s of pulled pork I did.

Add me to team @Darby_Crenshaw. Dry rub a day ahead of time and add more before throwing on the smoker.

 No worries re that fat in the pan. Water doesn't boil in a drip pan, so it never sees temps high enough to burn. 

Those black bits and sugary foam (if you mop sugary glaze on ribs) aren't ever actually going to burn with bitter smoke. 

They merely carbonize just like the black bark you get on bbq. No one imagines that the black bark smokednor created off flavors, but they see the same thing in a pan below it and assume the worst

fat won't smoke anyway, not til you are past it's smoke point. And that ain't happenin at bbq temps (even with the 'radiant heat from below' that is often cited)

One less thing to worry about. 

Am I the only one the uses mayonnaise?  Dukes brand of course.  I think it makes for better bark.