You're right. Many times, it boils down to a style preference. It's not like the "laws" of grammar will hold up in court.
However, please remember a few things.
First, there are writers here at various stages in their development. Many of them don't know a lot of the basic rules to begin with, so it's my intent to stick mostly to a good idea/bad idea perspective and not clutter things up with too many exceptions. In a lot of the instances we're encountering as we're validating fics, the semicolon wasn't used properly to begin with, be it inside quotation marks or out. I'm totally into discussing the virtues of various language conventions and the exceptions that go along with them, but that may be more appropriate in a different thread than one in which I'm trying to help people out who really don't know what to do. Yes, it's also good for the more advanced writers to brush up as well, and those of you who know the rules also know how to effectively break them or when it's a matter of personal preference--but that's not my primary purpose here.
Second, you have no idea how many times the phrase "you can't say a semicolon" has come up in my life just in the last week alone, between the aforementioned professors (from two totally different departments in the school, mind you) to experienced zine editors who've been doing the fanfic thing in print since the early 1980s. People who don't know each other, people who will probably forget more about how language works than I'll ever know, saying the same thing, almost verbatim: "you can't say a semicolon."
Third, some of the points you and Alelou bring up are for other lessons for other times. Just because I didn't bring it up in that post doesn't mean that I forgot or that I'm wrong or that I'm failing to look at it from a certain angle. it just means that I can't tell people everything they ought to know about writing dialogue in one post.

Nor should I try--it would be too long if I did and people would get frustrated and lose their patience with me. So I'm trying to keep it relatively simple.
Let's think about something else. If you're writing a line of dialogue that's so complex and convoluted that you feel compelled to use a semicolon for "organizational purposes", chances are you're going to get dinged by your beta reader, editor, teacher, admin, or whoever, on the grounds that "nobody really talks like that"--which, when you think about it, is another way of saying "you can't say a semicolon". And by "really talks like that,"we mean both in "real speech" (what real-life people actually say on a day to day basis) and "natural speech" (speech written either in literature or screenplays that isn't "real speech" but is meant to sound/read like it *is*). But again, that was supposed to be another lesson for a future time.
Okay, maybe you could justify a character's speech being so complex that you need a semicolon in a monologue, a la Shakespeare, or something like that. Sure. But the operative word in that scenario is
monologue. As in one person, who can go on and on for a while and say whatever they want without interruption. it won't have the natural flow of conversation any way.
Dialogue, by its very nature, requires two or more participants. I can't recall a time in real speech or natural speech when I've encountered any person or character who said something that would make me say, "Oh, snap--that should really have a semicolon!" Like many things in life, just because you can do a thing, that doesn't make it a good idea, and it's my considered opinion (as well as that of several college professors, editors of print fanzines, and authors of fiction writing style guides, apparently) that putting semicolons in dialogue is one of them.

While it's not necessarily wrong to do it, there's not a lot right about it, either.
And that's my opinion.
This also could be one of those things where we're witnessing the process of a change in language conventions--something that was considered acceptable before could simply be losing favor as these days the focus is on leaner, more streamlined expression in most situations.
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