I just lay ribs out on the counter and apply the rub to one side and let them sit for 5-10 mins. then flip and rub the second side. The rub will form kind of wet paste that will stay on the ribs very well. You can then wrap and refridgerate or smoke either one. This will work for a butt or anything else and will also allow you to apply another coat of rub if you wish. (double-rubbed)
Most cooks that use mustard say you can't taste it, can't see it, and can't smell it after the cook. If that's the case then why use it? You can definately gain rub adhesion and bark without it.
I use the mustard and then the rub. You can use many things to help with the adhesion...evoo, pam, etc. but mustard is the most common one that I know of. And like several members stated, you can also rub with just the rub. All a matter of personal preferance.