Though I frequently hear statements of faith in McCloud's definition and the propagation of it. However, I have still not received any good argumentation for why "comics" equals "sequential images with/without text." Frankly, this hasn’t surprised me, since I don’t think its possible to reasonably make such a claim.
Most people, I assume, are arguing this definition by following McCloud’s lead. However, at least in Understanding Comics, McCloud never provides an argument for his definition of "comics" either. Rather, he takes Eisner’s abstract notion of "sequential art" and then (as Horrock's first noted) recasts it as the definition for "comics." The reasoning for this follows no explicit argument, reasoning, or logic, McCloud does this solely out of preference stating,
"At one time or another, virtually all great media have received critical examination in and of themselves. But for comics this attention has been rare. Let’s see if we can rectify the situation. Eisner’s term seems like a good place to start."
And from here he begins to construct his definition around the base of "sequential art."
But, notice that from the very beginning he assumes that "comics" are a "media" to begin with, on part with "written word, music, video, theatre, visual art, and film." When separating "form from content" he assumes that "comics" are the form, not content. He begins the discussion with his position already loaded to believe that "comics" are a mode of expression, not simply an object that uses a mode of expression. He doesn’t say that sequential art is the medium that goes into the object of "comics," he makes them into the same thing.
There is no argument here for why "comics" should equal "sequential images," it is just a definition that is constructed out of the already stated assumptions that "comics" is some kind of medium.
This all has also got me wondering when "comics" as abstract notion first started emerging. Is it attributable solely to McCloud? This would be the usage of "comics," a plural, as a singular. Suddenly, instead of just being a type of book, it is able to be a medium or mode of expression (or even a type of scholarship – with far reaching implications here).
For instance, people talk as if "comics" was some sort of overarching category that subsumes manga, graphic novels, comic strips, bande desinee, etc. — "oh, they’re all just 'comics.'" Contrast this with "graphic novel." We don’t project "graphic novel" as an abstract; it’s a thing – a type of book.
If you reject the abstract formalist view in favor of "comics" only as a social object, these labels become more distinct in their own right. Graphic novels aren’t just a "type of comic," they are a format and literary movement distinct from comics. The same goes for manga, though it has even more slippery issues signifying both native Japanese works as well as a burgeoning OEL community.
The interesting thing I find in Jenkins' writing is that he heavily focuses on the associated social context of comics, while conversely saying they are a "mode of expression" that cuts beyond cultural context.
Again, to call "comics" a mode of expression misses the point. The mode of expression is drawing "sequential images with/without text" (aka "visual language" combined with "written language"). It is this mode of expression that is used within comics... and graphic novels and manga, etc. Though if you think you can prove otherwise, I'd love to hear the argument for why.