The greater density of the butter is supposed to keep enough pressure on the meat as it rests to prevent liquid from seeping out. Even a properly cooked and rested protein will have some liquid loss during the resting process (as evidenced by the pool of meat juices on the sheet pan of our resting racks after a busy nights' service). Like I said, the density and slight pressure from the fat is supposed to keep even more of this juice in the meat while it rests.
Also, as a secondary benefit, it coats the meat in a film of delicious butter. Most meats will have to be drained on a c-fold to prevent butter pooling on the plate, but there will still be a coating on the meat as it is served.
Demi glace, since it is water based and not fat based, would have no such benefit to resting the meat (preventing liquid loss). However, the resulting demi glace at the end of the night would be delicious.
Nikola, not every kitchen "worth a damn" uses demi glace. I know many fine Italian, Mexican, Asian and Middle Eastern restaurants (among others) that don't rely on demi for anything. In fact, I work in a kitchen that doesn't use demi. We get along just fine, thanks.
I've never done this in large scale volume before. I've done it for dinner parties and such, but never for a restaurant kitchen. The results were very good. I didn't do a side by side comparison, but I will tell you that the 2 times I've done it before I liked the results.
The way to prevent "washing off" of seasoning is to season your meat sufficiently before hand to allow the salt to penetrate the meat. See: Judy Rodgers and the Zuni Cafe Cookbook. It is the same idea behind pre-salting proteins for service ahead of time. Season all your airlines, all your t-bones, etc a day before they are to be served, which results in a spectacular steak/chicken.
I wouldn't do it with fish though...just meat proteins (duck, beef, chicken, lamb, etc). If anyone tries to tell you that pre-salting will make the meat dry then they don't know what they are talking about. It's bunk.
Just season like you would if you were about to put it on the grill or in the pan. Also, a bit of Malden or fleur de sel on the meat after it is sliced will go a long ways as well. I would keep the butter at about 115-120F for resting. The fact that it is being submerged in a "liquid" will help the meat cool faster than it would if it were resting in the air.