Ian,
For the record, I put myself through college working golf course maintenance, and even though things have changed a lot in those 30 years, I know something about it in my next career.
I think Jerry is right to call lacy edged bunkers trendy. At least he didn't call them "fads" which is, IMHO, a much shorter term connotation.
My point was that the regular shapes evolved because of maintenance technology, and you seem to agree. It was and probably is cheaper to smooth things out to the radius that a mower can mow. I also think that culturally, with our fascination with lawns and the formalized English landscape replicas in most suburban lawns, that the previous generation came to equate perfection (including bunker edges) with better design and maintenance. As you point out, that cut both ways - there is need to reestablish edges of all types. There is also a possilbe need to water bunker banks separately from fw and rough banks because of different watering needs, which is not edge dependent.
If there was a trend from 1930-1990, it was to look for ways to save money because golf was a crappy business in most cases. The trend towards great design at nearly all costs started when the economy ramped up about then. So, its a shorter trend than the frugal one, IMHO. Now, if you want to talk about what "bunkers are supposed to look like" there is an argument on another thread saying you ought to go back to the 1500's for that! No fair pickikng and choosing your time frame when discussing what bunkers ought to look like!