I dislike disorganized, poorly structured, or meandering meetings. Poor meetings treat the participants disrespectfully by wasting time and energy.
Yet, today I consciously chose to enter a meeting unprepared. I made that decision because the preparation for this specific meeting (a technical estimation meeting between a very few well aligned, frequently interacting, highly skilled people) would have used their time even less efficiently than performing that preparation during the meeting itself.
More typically I've felt the other way, that there were plenty of more effective ways I could have prepared for those meetings than spend the time in the meeting doing the preparation. This was a special (and rare) case.
I suspect there are "investment" heuristics which can be applied to the subject, the participants, the duration, my role, and the relative business value of a meeting. I envision the heuristic would provide a suggestion of how much time I should spend on the meeting based on those attributes. Unfortunately, google search did not show me that heuristic, or even a list of attributes of meetings which would help me decide how much to invest in preparation.