The popular question
Regarding the `book`, `report`, and `article` document classes: what are the main differences?
collects many differences between the book, report and article classes. In particular, some commands are not available in articles (like \chapter) while others are not defined for books (like \abstract).
What is the reason for these differences in functionality/availability of commands?
Some details: It is obvious that the current system has some disadvantages if one likes to switch from one classes to another one. For example, 'downgrading' a report with chapters to an article does not even compile and needs some manual work (or workaround). On the other hand, I can see that certain commands don't apply literally to all kinds of documents. In an article one would probably not speak of chapters or include back matters. Nonetheless, viewing the corresponding commands just as a way to structure the document does make sense. That is, there is no problem in regarding \chapter as the highest level heading and \front|main|backmatteras a way to tell latex 'now comes the following thing, so maybe change the formatting accordingly'. Of course the styling and effect of these commands have to be different in an article vs book.
So the refined question is: what is the advantage of the current classes versus a base class providing the same commands with an option for the style.
articletobook; I see no reason for usingreporteither. – egreg Aug 29 '15 at 14:27\mainmattercommand just turns on sectional numbering ..." – Mike Renfro Aug 29 '15 at 15:57\chapter. For me articles don't have chapters, and I don't think if is good markup to use the word\chapteronly because it make some coding easier. If you need switchable commands define some wrapper command, e.g.\let\topsection\sectionand use this. – Ulrike Fischer Aug 29 '15 at 17:18amsprocandamsartis nearly identical; the main difference is in the styling of output. (neither is compiled into the larger unit in a single run, since different authors nearly always use different packages; the "collecting" is done separately from pdf files.) – barbara beeton Aug 29 '15 at 18:46