I like mustards, of all kinds. Grew up with too many hot dogs at too many ball games I guess. So still like mustard on lots of things and occasionally use it as a pre-rub.
I agree with the others, it has little effect on the actual final taste but is a good binder for your actual rub. And, it does seem to give you a thicker Bark. Certainly a Bark with a somewhat different texture than you would get without it.
One thng I think about when I am deciding to use mustard or not, is the ingredients in the actual rub that I am going to use. For instance, if I am using Jeff's rub or a rub of similar nature, I might lean toward using mustard. Those ingredients are great whether alone or in combination with the mustard. But, one of my favorite flavors is a garlic, sage, herb based rub. When I use these ingredients, I usually rely on EVOO. I just like those flavors so much I don't want to risk them being covered up or masked, even if it is only minimally.
Bottom line is, even if you don't like mustard, you have little to risk. The mustard is so benign, it will hot hurt anything. Like the others say, you owe it to yourself to try it once, Just so you can speak from experience and not from worry. I think you will be glad you did.