First, great topic!
Second, the answer is "it depends."
Third, just last week, I was called in to do a bunker reduction study, but in fact, the super didn't want to reduce the few very large bunkers the course had, but rather, enlarge the small ones, by "connecting" a few nests of pot bunkers for easier raking, which surprised me a bit.
I have designed the occasional "nests" of small bunkers, often one on an fw complex, and another by a green. My reasoning is that most bunkers on any given course look too similar. And, particularly in mounds behind greens, I have had supers tell me that its easier to mow the turf surrounds if the mower can just keep going up or down the hill, rather than have to turn on a slope.
I read an article on energy consumption in the Sunday paper which made an interesting point. Many feel the way to be energy independent in the US is to keep drilling for oil, ignoring the fact that if we reduced energy consumption first, there would be less need to dig. That's not unlike dieting....if you don't eat the calories, you don't need to worry about burning them off. And, not unlike designing in reasonable maintenance budgets, i.e. bunkers cost a lot to build and maintain, so designing fewer of them is the first step in a financially sustainable operation.
That and some old quotes from Ross tend to make me think that for most courses in the "fun to play every day at a reasonable price" category, that the gca is best served to keep sand bunkering to a minimum that meets the design needs of the course. Of course, a resort needs some features average golfers don't see every day, or they won't drive/fly very far to experience the course. Similarly, every town probably has one course whose mission is to be the hardest in town, and they probably want more bunkers.
Bunkers can serve many purposes, i.e., hazards, plus direction (or misdirection) devices, aesthetics, safety, separation, drainage, and to save shots, to name a few. I tend to place bunkers only where they serve at least two purposes, and almost never build a blind bunker (i.e. Ross again, making it the players job to avoid it) but also because they cost so much to build and maintain, it seems a shame to NOT see all that money spent on enhancing your enjoyment of a golf hole.
Lastly, bunkers are sort of like the number of employees in a large corporation, where it is said you could probably eliminate 20% of them and no one would notice.....