I see this as a very encouraging trend. I'm not sure that I would want to book one of these cruises, but many of the complaints and conflicts that I see in these forums where cruising is discussed relate to the cruiselines trying to be all things to all people. There are people that want to dress for dinner, people who don't, people who want to bring their infant, people whose worse nightmare is having tablemates with an infant, people who smoke, people who hate smoke, people who enjoy hearing Who Let the Dogs Out once every two hours, etc.
As long as the cruiselines try to attract all of these people onto the same ship there are going to be problems. But there are so many ships and sailings that the cruiselines can afford to attempt to attract more of a niche group onto a specific sailing. If there is a market for it, it will succeed. If I am uptight about tieless people in Dockers on formal nights I might just seek out a cruise which the cruiseline promotes as being more formal. If you don't want to be confronted with somebody dipping their diaper clad darling in the hot tub, you might want to book one of these Adults Only cruises.
Exclusionary, sure, but we all want to "get away from it all" and maybe, just maybe, some of us are precisely what some others of us are trying to get away from.