I subscribe to certain substack writers. One very annoying thing is when the author starts introducing guest posts or worse, subscriber contributions. The point of paying money to read someone's writing is to, well, read that person's writing and not the writing of other persons and especially not the writing of other readers.
Now, I realize that if the person I subscribe to consistently writes, say, 10 articles a month, then it doesn't lessen the value if that person posts a few other articles in addition to those 10. I'm getting 10 articles regardless.
But sometimes, it seems like inviting reader submissions will enable the substack author to offer only 9, or maybe 8, articles a month. I haven't done a study of the small number of substacks I subscribe to to know if that is actually the case. I suspect it is, but I haven't been counting the articles.