I may have discovered an answer to my puzzlement, if not THE answer, in Andrea Costadoro's 1856 work:
"It has been a matter of discussion what books should be considered pamphlets and what not. If this appellation is intended merely to refer to the SIZE of the book, the question can be scarecely worth considering ; but if it is meant to refer to the NATURE of a work, it may be considered to be of the same class and to stand in the same connexion with the word Treatise as the words Tract ; Hints ; Remarks ; &c, when these terms are descriptive of the nature of the books to which they are affixed." (p. 42)To be on the shelves of libraries, and cataloged, it is possible that these pamphlets were indeed bound, perhaps by the library itself.
The Library of Congress genre list today has a cross-reference from "pamphlet" to "Tract (ephemera)". While Costadoro's definition doesn't give any particular subject content to the type of work, LC's definition says that these are often issued by religious or political groups for proselytizing. So these are pamphlets in the sense of the political pamphlets of our revolutionary war. Today they would be blog posts, or articles in Buzzfeed or Slate or any one of hundreds of online sites that post such content.
Churches I have visited often have short publications available near the entrance, and there is always the Watchtower, distributed by Jehovah's Witnesses at key locations throughout the world, and which is something between a pamphlet (in the modern sense) and a journal issue. These are probably not gathered in most libraries today. In Dewey's time the printing (and collecting by libraries) of sermons was quite common. In a world where many people either were not literate or did not have access to much reading material, the Sunday sermon was a "long form" work, read by a pastor who was probably not as eloquent as the published "stars" of the Sunday gatherings. Some sermons were brought together into collections and published, others were published (and seemingly bound) on their own. Dewey is often criticized for the bias in his classification, but what you find in the early editions serves as a brief overview of the printed materials that the US (and mostly East Coast) culture of that time valued.
What now puzzles me is what took the place of these tracts between the time of Dewey and the Web. I can find archives of political and cultural pamphlets in various countries and they all seem to end around the 1920's-30's, although some specific collections, such as the Samizdat publications in the Soviet Union, exist in other time periods.
Of course the other question now is: how many of today's tracts and treatises will survive if they are not published in book form?