The one architect I know of today who doesn't believe 18 holes are necessary is Edwin Roald, who's based in Iceland, where they don't care so much about the tradition of eighteen. He has built a bunch of courses that are 9 or 12 or 14 holes because that's as much as he could fit onto the good parts of the land, and it made no sense to spend the money to get four other holes up to standard out of a difficult situation.
To the last question just posed by Padraig, it's very hard for most architects to even think about building less than 18 holes . . . it's just our mission statement when we start to find the best 18 holes we can. And yes there are always a couple of holes that seem weaker than the others that you know you're going to have to work harder on, but you never think about just stopping at 15, or at least I don't. I would, if the client said right from the beginning that was fine with him, but I would never try to talk a client into that. It's just too far out there for most people.