And I still remember the response to that being "because we need to bear witness on behalf of the canon". I think that's also a valid reason to physically go in rather than MST at a distance and then pop in at the end -- because the agents need to see firsthand what the consequences of the badfic are on the canon.

I am pretty sure that's the best "real" answer, since I don't recall a specific explanation being given in TOS, Intelligence, or any of the LJ supporting docs. Architeuthis says that she must sometimes enter a fic she's observing because the negative effects might not be obvious just from reading it, but doesn't bother explaining why that is. It's just taken for granted that the agents must go on missions, because that's the story.

For me, though, I'd suggest it's a combination of a couple factors. First and foremost, making a decision to assassinate a target at a distance feels cold and skeevy to me. It would be much too easy to become callous to the possibility of OCs being salvageable if the agents saw their job as simply finding things to be annoyed about in the text, then jumping in and removing the annoyances as quickly as possible. That's how you end up with the Flowers simply nuking any world believed to be contaminated with Suvians. Or you might get the opposite problem: agents deciding bad things aren't really that bad after all, because if they flag it, they have to deal with it later. Either way, it's no good.

Also, as Archi suggests, there's a difference between what the Words say and how they affect the continuum, a.k.a. tell vs show. From the reader's perspective, a good mission shows the effects of bad writing; that's what makes it different from an MST or plain review. From the agents' perspective, it's much the same: reading a thing doesn't necessarily tell you how bad it is, because the Words don't always have the same effect from fic to fic (partly because people handle the same thing in different ways in their missions). Therefore, it's not as simple as scanning a story for figurative language and other stylistic choices that can cause reality distortion. Some things that might be fine in an otherwise-decent story may be interpreted literally in a story that doesn't know what it's doing, and vice versa, so you have to get in there and see what actually happens in this particular fic. And that's why agents must observe the charges for them to be valid: you can't know for sure how a bit of text is going to manifest until you get there.

And it allows the agents to observe indirect consequences of a fic, such as Archi discovering the Witch-king can read English while talking to him in Minas Morgul, or Jay seeing Legolas wandering around in a trance murmuring about beautiful Laurel. You don't get that from reading the Words. (Out-of-universe: more showing! Storytelling possibilities! Our narratives don't have to follow the fic in lock-step the whole time!)

And narrative entanglement, and disrupting a Suvian's control over events, and whatever other explanations help the agents accept their jobs.

Though, I don't think the earliest agents needed help. Jay and Acacia and their contemporaries did it because the perks of the job—actually getting to visit your favorite fictional worlds and see your favorite characters up close!—were reason enough. Those are the rules, huh? Great! No explanation required. {= )

... In the end, when your premise is this meta, you can't disentangle the meta logic from the in-universe logic. It's all of a piece. And it's more fun that way.

~Neshomeh