I have read thousands of questions on this forum, and so far answered several hundred. This gives a little insight into how various users think... and given how many questions on MATLAB Central Answers can be answered by simply referring to the (very first line of the) inbuilt function documentation, it seems that your question "do casual users of FEX know...?" is sometimes going to have to be answered in the negative: if many "causal users" can't even find their way to the first line of the official documentation for the inbuilt functions (which also come at the top of [name-of-search-engine] results), then what is the chance of them finding their way past some little lightbulb icon?
But I would argue that the majority of users find documentation useful: if your functions are well written and well documented, than this will be appreciated by the ones who know, understand and who might find your submissions useful, both learners and experienced alike. The publishing feature is very handy for creating self-documenting examples, and I think it is better to not worry about the lowest-common-denominator, but to keep your focus on providing the best documentation and explanations of your work: this is always appreciated by other users, even the learners and all of those who don't leave a comment.
From my experience I certainly appreciate example files, and like that authors can take little more time (and space) to explain the thinking and requirements for using their submission. And it is always useful to have something to refer to when working with a new function, so I think there is certainly value in these example files.
And of course pretty pictures always helps sell a product!