Many people have strong opinions on when to apply a rub — some say to allow at least a few hours and preferably overnight. Others literally apply it as they are putting them in the smoker/oven. Since people do it both ways and both claim to end up with terrific, moist ribs, whatever effect this might have is probably small.
It may change aspects of the “glazing” effect that a rub might give, depending on what is in the rub. A rub left on for a longer period will become more mushy and more like a paste or a glaze/sauce, while a rub put on fresh before cooking won’t have time to become as moist. You will get slightly more flavor penetration with a longer rest, but there are diminishing returns there too after the first hour or two.
I don’t have experimental evidence to back this up, but I think the “early rub will dry things out” argument is pretty flimsy from a food science perspective. It is true that rubs which contain significant amounts of salt, sugar, or other hydrophilic substances will cause some moisture to come out of the meat. On the other hand, most people tend to cook ribs for quite a few hours anyway (and some smoke them for many hours), so the rub will have plenty of time to draw out that moisture regardless of whether you put it on ahead or time or right before cooking.
It’s not like you’re salt-curing the meat and leaving the salt on it for months. Once the salt (and/or sugar) draws out the moisture from the outermost thin layer of meat, it generally takes much longer for moisture to migrate from inner parts of the meat. That surface layer of meat will release most of its moisture within an hour or so after you put salt or sugar or whatever on. So, even if you put your rub on immediately before putting it in the smoker, that moisture will tend to be drawn out in the first part of the cooking time. Adding a few more hours or even overnight to the rub shouldn’t result in significantly more moisture loss from the meat’s interior — and the exterior will always dry out a bit as it cooks anyway.
Moreover, you aren’t generally draining away moisture from the surface. With ribs, you often have them wrapped up to rest, which means the meat sits in that moisture. And guess what? About 10-15 minutes in, the brine produced by the moisture combined with the salt in the rub will begin to break down the outer muscle structure of the meat and cause it to absorb more moisture than usual, so much of that liquid lost will be reabsorbed back into the meat within an hour or so. (Note that the salt and water are carried back into the meat with some of the other spice compounds which are in the rub, so there is perhaps some flavor advantage to applying the rub at least a little in advance. On the other hand, again this mostly affects only the outermost layer of meat, so more than an hour or two probably won’t make more significant changes in the flavor.)
If you’re talking about searing a steak, when you salt it might have some impact, since you generally don’t want to time your sear at the point at which moisture is being released at the maximum rate. But for ribs or anything with a long slow cooking time? It’s really not a big issue.
As mentioned above, the larger effect will generally be what happens to the rub as it sits on the meat for many hours. You may find that the change in texture there may change the final appearance or texture of the outer surface. But the idea that the interior will dry out significantly? It doesn’t seem likely.