As the longer and longer post-doc becomes more and more standard for us, I would argue that there's more of a reason to do a quick and dirty dissertation. Because as the standards for post-doc TT applicants rise, (I feel like) there's less and less benefit to doing something impressive as a graduate student. Getting high profile publications is essential for a post-doc, but beyond some low threshold, doesn't matter as much for a grad student. Not as much as it did 25 years ago, I would imagine. I've heard people who have votes in faculty hiring say that they discount grad student C/N/S publications, since they are thought to reflect mostly the PI. And even if this is wrong, who cares, since the grad student isn't going to be applying for a faculty position for several years, and it's the most recent papers that matter most (the "what have you done for me lately?" principle).
When you apply for a post-doc, it mostly comes down more to whether you seem smart, and give a good talk, and have good letters. When you apply for a F32, my impression is that the reviewers discount impact factor in favor of just having a few reasonable pubs.
My hunch is that a job applicant with big papers as a post-doc and nothing as a grad student has a much better shot at that faculty position than someone with the reverse record.
Maybe the dissertation is becoming more of a pre-post-doc. The goals are to learn the skills, and to begin to form the friendships, that will serve one as a post-doc. What's the incentive not to just get through it as quickly as possible? [And I realize many people don't want to stay in acedemia - but I think these arguments apply equally well to them: why wait to get started on the preferred life path?]
These are the thoughts I had near the end of grad school. Due to various reasons, I was faced with only two options: graduate in 5 or in 6. Had I gone with 6, I would have likely had another pub, but I reasoned, that pub was not in the field I wanted to pursue, and wouldn't mean much for me professionally. I left in 5 and tried to get the pub out from long distance, but it's still in limbo, 7 years later. And here's the thing: I don't regret it at all. I was sick of grad school, I was eager to go do more interesting stuff, and I did. And as a post-doc I got paid more.
So I wrote a crummy dissertation at top speed (still ~3 months, I'm a slow writer), and one of my committee members (who I adored) told me I would never succeed in science if I continued to write like that. "But," I thought "I can write better, I'm just doing a poor job because no one cares what I write in my dissertation." And as far as I know, he was the last person to ever glance at my dissertation.
I realize that everyone is in a slightly different situations and there are going to be large numbers of exceptions to my generalizations here. And I realize that I am overgeneralizing from my own experiences. But I have several friends who are reaching the same decision point I was at in my 4th year of grad school. And many of them, unlike me, have perfectionistic tendencies, and desires to write an A+ dissertation. And in most cases, I am gently nudging them in the direction of just getting it done and moving on. I think that would make them happier, and would most benefit them professionally.