How important is your written PhD thesis for an academic job search, assuming the results are already written in papers? Do search committees ever read your thesis in addition to your papers? My PhD advisor said that the written thesis is not important and I can write whatever I want as long as it's not about Star Wars.
Your thesis and what is written about it your letters of recommendation will have a big effect on your career initially. To get a good postdoctoral position, you'd better have a good thesis. Later in your career, your thesis won't matter much any more, because people will be more interested in your recent achievements.
I am a faculty member who sits on many PhD thesis committees. I always read the thesis. More than once I (and other committee members) have decided to fail a student at the thesis defense because of a thesis that was poorly written. This usually was a combination of both small issues (like rampant grammar and spelling errors) and larger issues (like lack of clarity, lack of motivation, no clear indication of the student's contributions). Usually these students rewrite the thesis and retake the defense successfully later.
You cannot fully separate the quality of the writing and the quality of the work. Excellent work can be masked by poor writing; poor work can be made to appear good superficially by superficial (hence poor) writing. Surprisingly often I find that as one digs into the writing issues, one finds significant scientific issues.
Even if your institution and/or your advisor advertises low standards and is willing to graduate you with a poorly-written thesis, it is probably in your best interest to produce a well-written thesis. Writing is one of the central skills that will get you an academic job, and writing a good thesis will help substantially in the development of that skill. At this point you probably have few papers, so it is likely that potential employers will look at your thesis. It is often the first thing I look at when hiring a postdoc, and if it is poorly written then I assume I will need to spend a large amount of time helping that person learn to write and editing their work -- which usually means I won't hire them.
Finally, a well-written thesis should be wonderfully satisfying to you. Your thesis should be your magnum opus, a thorough explanation of this new corner of knowledge in which you are the world's leading expert! Put in the effort to do it well.
This will depend on your field, and how your thesis is formatted. That being said, I don't think your advisor's (hopefully) flippant answer is correct. If nothing else, while the content of your thesis might not be read, your thesis topic is the thing you're best positioned to present yourself as The Expert on, and as such will have a pretty significant impact on your job search.
You will, on the academic job market, be expected to talk about your research, etc.
Beyond that, many people repackage their thesis as a series of papers - indeed, many institutions now allow (or require) a thesis to be formatted this way (sometimes called a "sandwich thesis"). In this case, you're dividing your thesis into several hopefully published papers, and you should absolutely plan on those being read.
As for your academic career, it's not the most important, but it's not unimportant either. A sloppy thesis gives a bad impression about you, even if the research itself is good. It's true that if you did a "sandwich" thesis, the majority of people will only read your published papers and not your thesis. But not all of them: think of the people on your thesis committee and the future members of your research group for example, do you want them to have a bad impression about your work?
Besides, there is one central person that you need to please when writing your thesis: yourself.
A PhD thesis is something for which you put tremendous amounts of effort, personal commitment and sacrifice (salary, free time, etc.). You should be happy with the result. You might browse it in the future and that might annoy you that such good research is presented so poorly.
One week, counting the nights, is a lot of hours, enough to turn an ugly draft into something you will be proud of in the future.
From my experience, at least with regard to life sciences your advisor is absolutely correct. In life sciences, journal publication is the standard and everything else is typically considered "unpublished" and "unreviewed" - and most people will not bother reading it.
The only exception I can think of is if you have important material in your thesis that is not published yet (for example if the project is still ongoing). Even then, you would have to specifically point out that this is the case and urge the relevant people to look in your thesis (and it may still seem weird).
Of course I am not passing judgement whether this state of affairs is good or bad.