I don't know how much a DC ticket costs over here, but I'd have been mighty annoyed if I'd shelled out good money for one and then only seen someone trounce somebody else 6-1, 6-1, 6-1, say. Thinking of parallels with cricket, that's the sort of reason you often get poor turnout on the last day of a multi-day match, because people don't think it's going to be worth their while going to all the effort of getting there only to see maybe a few balls being bowled. I suspect it's not a very healthy situation for the sport to be in.
OTOH, if, say, Djokovic/Murray had been playing this weekend, I could have understood them not wanting to exhaust themselves further before their next scheduled tournaments.
If it goes 6-1 6-1 6-1 they have to play the final rubber. They only have a choice if the 4th rubber goes 4 or 5 sets, and even then both have to agree not to play, if 1 team wants to play they play. The fans will get at least 4 sets
They ought to play out all matches, dead or live. Put some young kids in the dead rubber, get some experience, while the crowd gets their money's worth.
Sure the top guys have all got 250's and challengers to get to, but every DC team should carry a couple futures level kids just in case.
Dead rubbers have to be played by one of the 4 players named to the team. Which is really stupid imo, the way it is I could careless about seeing some doubles mug get creamed in a dead rubber. If they made the rule so anyone could play the dead rubber then it'd be much better as young practice partners could play and get some experiance like you said