In my view, a Ph.D. dissertation is an over-rated document. I'll formalize my position here so I don't get dragged into endless debates during every thesis defense I attend :-).
A classic Ph.D. generally has a hypothesis that is formulated at an early stage (say, year two) and the dissertation describes 3-4 years of research that tests that hypothesis.
A modern Ph.D. (at least in computer architecture) rarely follows this classic recipe. There are many reasons to adapt your research focus every year: (1) After each project (paper), you realize the capabilities and limitations of your ideas; that wisdom will often steer you in a different direction for your next project. (2) Each project fixes some bottleneck and opens up other new bottlenecks that deserve attention. (3) Once an initial idea has targeted the low-hanging fruit, incremental extensions to that idea are often unattractive to top program committees. (4) If your initial thesis topic is no longer "hot", you may have to change direction to be better prepared for the upcoming job hunt. (5) New technologies emerge every few years that change the playing field.
I'll use my own Ph.D. as an example. My first project was on an adaptive cache structure. After that effort, I felt that long latencies could not be avoided; perhaps latency tolerance would have more impact than latency reduction. That led to my second effort, designing a runahead thread that could jump ahead and correctly prefetch data. The wisdom I gathered from that project was that it was essential to have many registers so the processor could look far enough into the future. If you had enough registers, you wouldn't need fancy runahead; hence, my third project was a design of a large two-level register file. During that project, I realized that "clustering" was an effective way to support large processor structures at high clock speeds. For my fourth project, I designed mechanisms to dynamically allocate cluster resources to threads. So I had papers on caching, runahead threads, register file design, and clustering. It was obviously difficult to weave them together into a coherent dissertation. In fact, each project used very different simulators and workloads. But I picked up skills in a variety of topics. I learnt how to pick problems and encountered a diverse set of literature, challenges, and reviews. By switching topics, I don't think I compromised on "depth"; I was using insight from one project to influence my approach in the next. I felt I graduated with a deep understanding of how to both reduce and tolerate long latencies in a processor.
My key point is this: it is better to be adaptive and focus on high-impact topics than to flog a dead horse for the sake of a coherent dissertation. Research is unpredictable and a four-year research agenda can often not be captured by a single hypothesis in year two. The dissertation is therefore a contrived concept (or at least appears contrived today given that the nature of research has evolved with time). The dissertation is a poor proxy when evaluating a student's readiness to graduate. The student's ability to tie papers into a neat package tells me nothing about his/her research skills. If a Ph.D. is conferred on someone that has depth/wisdom in a topic and is capable of performing independent research, a student's publication record is a better metric in the evaluation process.
In every thesis proposal and defense I attend, there is a prolonged discussion on what constitutes a coherent thesis. Students are steered in a direction that leads to a coherent thesis, not necessarily in a direction of high impact. If one works in a field where citation counts for papers far out-number citation counts for dissertations, I see little value in producing a polished dissertation that will never be read. Use that time to instead produce another high-quality paper!
There are exceptions, of course. One in every fifty dissertations has "bible" value... it ends up being the authoritative document on some topic and helps brand the student as the leading expert in that area. For example, see Ron Ho's dissertation on wires. If your Ph.D. work naturally lends itself to bible creation and you expect to be an ACM Doctoral Dissertation Award nominee, by all means, spend a few months to distill your insight into a coherent dissertation that will have high impact. Else, staple your papers into a dissertation, and don't be ashamed about it!
My short wish-list: (1) A thesis committee should focus on whether a student has produced sufficient high-quality peer-reviewed work and not worry about dissertation coherence. (2) A dissertation can be as simple as a collection of the candidate's papers along with an introductory chapter that conveys the conclusions and insights that guided the choice of problems.