fat side up can help keep things moist as the rendering fat sort of bastes the meat as gravity pulls it down into, and across the surface of, the meat.
Ziggy, my friend, the "fat side up to baste the meat" is a myth. It's the same type of anecdotal thinking as "meat on the counter for an hour will reach room temp." No, it won't.
If I owned a horizontal offset without tuning plates, the heat would come from the top, not the bottom, unless I put the meat right next to the firebox. Away from the box, fat side up. Near the box, fat side down. Tuning plates, fat side down anywhere in the smoker.
I've always put the fat cap toward the heat source and get tender juicy briskets, tri tips, and other fat capped meat. If there was any truth to the "fat side up to baste the meat," that would not be possible.
Yes, the fat cap will render with the fat side up, but it will flow around the sides and into the drip pan. It is physically impossible for the lower density external fat to flow into or through the denser meat muscle.
External fat has nothing to do with making the tough meat juicy. That's melted collagen's job, and internal fat, to a degree. As the tough collagen melts, the meat becomes tender and juicy. A tender lean and juicy flat is proof. An undercooked flat is dry and tough.
Rendered internal fat will turn butterlike as it heats and the collagen melts. It remains in the meat and adds flavor, texture, and yes, juice. Think waygu, Prime, or any brisket point from Select on up. If you've ever undercooked a brisket and chomped on an unrendered point, that greasy feel in your mouth is one you won't forget.
We can agree to disagree, but you are absolutely right to tell people to try both ways. The engineer in me always tests cooking advice that I know to be anecdotal.
BTW, the same fat cap advice applies to pork butts, too, but that's another thread.