Casinos, particularly in LAS VEGAS, are "tip happy"--the whole culture there is tip oriented. In LV there are numerous publications (they are left in your room), noting "suggested" tips. In my OP the buffet I referred to is MGM (in the last month, they changed from wait staff bringing you drinks to SS).
At one time buffets were < $10, now (if its a champagne brunch) can be > $50.
If a wait person regularly clears plates from your table and keeps your champagne glass full (which would be the ONLY service they provided), a tip should be given. But, as I noted in my OP, they are no longer serving drinks.
PS: The intention of my post was not to start an anti-pro tipping discussion, but to get FTer's opinions on tipping with ONLY SS drinks.
PS(PS): At many buffets there are "stations" where food is specially prepared: omlets, Mongolian, crepes, pasta, fresh scooped ice cream, etc.--all of these EXPECT tips (they leave a few $ in a cup, prominently displayed)--but that is a separate issue.