The purpose of your thesis is to demonstrate to them (and perhaps the larger scientific community) that you can communicate to other scientist in the language of the field, that you possess the requisite knowledge, and that you are prepared to advance that field.
There is no guild, but there are gate keepers (reviewers of various sorts) and you must be prepared well enough to make convincing cases (for publication, funding, etc.) Your advisors were training you for this role.
Science is first and foremost about understanding, and writing in a way that purposefully obfuscates and makes it harder to understand what is communicated is anathema to true science.
While I fully agree that ability to communicate science to the general public is incredibly important, the thesis is not necessary the place for this. Plenty of other places are (blogs, twitter, etc) and this ability is crucial for a publicly funded scientist. General large conferences that I am aware of often encourage non-technical translations of abstracts.