Yes, I understand the institutional pressures to get students out and yes I understand that articles are where the payoff is, but I can still lament the loss of sustained attention to a single argument that used to characterize a much higher proportion of Economics dissertations. And a title such as "Essays on the Macroeconomics of Trade Flows" is singularly uninformative in helping me understand what this person might be writing about.
Yes, I sound like an old fart, but so be it. "Three Essays in Search of a Staple" has won out in the "competitive" process, but that doesn't mean I have to like it.
In the old days you learned how to do in-depth research first and, once that was mastered, learned how to get it published. The almost unconnected essays are the most frustrating and not just for grumpy reasons. You sometimes might wonder if the 3 essays breeds a lack of patience, a hesitation to fight through a difficult problem and an unwillingness embrace the minutiae. All of which makes for sloppy research (I should know, mid-career time constraints [sometimes*] force me into that situation). Experience allows you to learn where you can take shortcuts and where you can't (but maybe PhD programs work on teaching that these days?).
I wrote a 3 essay dissertation that explored facets of Major League Baseball's arbitration system. I chose this format for the familiar reason: because I wanted to have nearly-publishable papers ready to help me find a job when I finished grad school (I got three from the first two full essays). I don't see anything wrong with the three essay format as long as the papers are on a fairly-narrow subject and are somehow connected.
As far as slop and the lack of patience goes, those are two qualities that need to be hunted down and mercilessly killed, in a bloody manner if need be, if one is going to get a decent product out of the dissertation research. I had an advisor who wasn't going to let me finish without fully exploring the problems at hand. It was something that I didn't always like at the time but that, looking back, I'm glad I went through.