Bear, I used yellow mustard for the first time on baby backs and...I'll never go back to no mustard. After I put the rub over the mustard I looked at the ribs and I was finally looking at ribs the way I've seen them in pictures and on TV. The mustard held the rub onto the ribs really well.
However, I've got a question you might not have an answer for. I had two racks of very meaty ribs and I smoked them at around 225-240* (you know how the MES 30 controller can swing temps over time). For the first time, the meat on both racks split vertically over the thickest parts of each rack. I noticed this when I unfoiled the ribs to baste them with BBQ sauce during the last hour of cooking (I ended up doing a 4-1.5-.5 method which turned out to be right on the money for how I wanted the meat to turn out). Could it the mustard have contributed to this since it's never happened before? I have no idea which is why I'm asking.
Don't think I've seen that, but I don't think mustard would do anything like that. Not sure what you mean by vertically---You mean cracks across the bones?? Or between the bones?