On run-on sentences, Wikipedia says:
This is generally considered a stylistic error, though it is occasionally used in literature and may be used as a rhetorical device.
At the end of the article it describes how run-on sentences are used in literature. For example, some authors use them to depict stream of consciousness. Is that a rhetorical device or just an example of it used in literature? What valid uses are there for run-on sentences?
Best Answer
This is opinion based and maybe better asked at a writer's site.
James Joyce used run-on sentences to great effect in Ulysses, which, if I recall correctly, has the longest sentence in English Literature. Does the style need a raison d'etre other than that the author uses it skillfully?
If one is trying to recreate human thought processes (or conversation, for that matter), run-on sentences and sentence fragments are much more representative of thought/speech than 'proper' writing. Also, if one is trying to accurately portray speech patterns in certain types of mental illness, it would be far better to use run on sentences. We rarely think in well-developed sentences, although it is the better way to communicate our thoughts.
An encouraged use for run-on sentences is in certain types of journaling, in which one writes whatever comes to mind whenever, to uncover thoughts which might be otherwise be kept hidden.
It is not commonly encouraged in writing classes because it is often just laziness. When used purposefully, it is difficult to carry off well. Also, bad run-on sentences make reading hard work, whether one believes in an interactive or modular approach to reading.