"15 words" is a secretly a verb rather than a noun.
As the thread creator you are entitled to specify the way you want the phrase to be used and what sort of replies you want. That said, it seems that the norms that you are attempting to create and enforce for this '15 words' activity don't belong on this site. It seems to amount to provoking and enforcing all the worst of the failures of critical thought that constantly crop up in the "Rationality" Quotes threads. Given as a premise that I hold that belief you could infer that my voting policy must be to downvote:
I'm sad to some people engaging solely in that and missing an opportunity to try out the exercise instead.
That isn't what you saw. You saw people engaging in that in addition to engaging with the the exercise. They lost no opportunity, you merely couldn't tolerate the critical engagement that is an integral part of discussion on a rationalist forum.
People want to tell everything instead of telling the best 15 words. They want to learn everything instead of the best 15 words. In this thread, instead post the best 15-words from a book you've read recently (or anything else). It has to stand on its own. It's not a summary, the whole value needs to be contained in those words.
I'll start in the comments below.
(Voted by the Schelling study group as the best exercise of the meeting.)