Pat,
I would agree with you that rough holds balls near the green, different than the old Scottish courses, and that it was largely done specifically for that purpose to speed play, and keep maintenance costs down. It might also have been to keep the course tougher, as per the US Open mentality from 1951 on.
Then, in the '92 Open at Pebble, the USGA experimented with it for the first time (from memory, others can correct) and it slowly caught on from there. It might be a concession to both fun golf and older golfers, giving the finesse guys (who by and large are compensating for lost distance with more guile around the greens). So, even though it started out as an idea for variety in US Open set ups, this one might actually make sense for the everyday course, too.
As always, lots of stuff goes on at any point in history, and I might be missing things, but that is my general take on the situation.