TC, say is ain't so. You're going to another site for Q info...how could you.
This is one of those old smoking "secrets" that has been around for years. And why HAVEN'T we discussed it here?
I don't know that it adds to the bark in my experience -- maybe if you put it on REALLY thick. But as always, I say try it and decide for yourself.
Taste. Nope, you won't taste the mustard at all (actually the same can be said for the mayo on the chicken thing, you won't taste the mayo either). The is so little substance to mustard or mayo that most of it cooks off.
As far as why, many use it in the belief that it will help the rub stick better, or that the vinegar in it will help tenderize the meat. Maybe yes, maybe no -- who knows.
I've found that a rub, applied at room temp, will rather quickly, start becoming damp through contact with the meat and this will help itself stick. I let the rub sit for 10 min or so and THEN I sprinkle on more rub if needed.
Another option, that I do use, is to create what someone called a "slather". I add my rub to mustard in a bowl and mix it up well, then slather that on the pork putt. I let the mixture sit for a while then sprinkle on more rub.
and that's my Mustard story