Mustard is just used to help the rub adhere alittle better to the meats. I've used mayo on some things also. Some use a coating of EVOO. I usually just leave the ribs alittle damp from water before adding rub.
If I had a dollar for everytime I forgot the mustard I'd have 5 dollars.
You'll never tell the difference. I didn't put it on a picnic once, the only thing I seen there was the bark wasn't as thick.Hell I'm not really even sure that was the reason.