As a fairly heavy user of the em dash, I was please to see Erin Brenner 's defense of it at Visual Thesaurus (responding to a rather heavy-handed Slate article that called for abolishing the punctuation mark entirely).
But Brenner herself used one construction I think is odd, and relatively new (decades old, that is, rather than centuries). Yes, that may be the Recency Illusion at work. But I suspect her usage is more acceptable to younger speakers of English. The construction in question:
I won't argue that writers sometimes overuse the em dash.
What Brenner means is (in my dialect), "I won't dispute that writers sometimes overuse the em dash." It's the Slate writer who "argues that writers [do] sometimes overuse the em dash"; Brenner is countering that argument. And for me, "to argue that" means only "to state the case for" something -- not to argue against it. The OED seems to agree, since there's no definition or example of "argue that" meaning "dispute that" or "argue against the notion that."
Judging from a quick look at Google News, it looks as if "argue that" for "dispute that" generally appears in the negative form -- "I certainly would not argue that providing the opportunity for someone to take time off if they're not feeling well is a good idea," for instance. So maybe the people who use it assume that the not is enough to reverse the sense of "argue that." Not for me! "She argues that the em dash is overused" means "she makes a case for its being overused"; but "she doesn't argue that the em dash is overused" means she doesn't make a case for its being overused -- not that she doesn't dispute the claim of overuse.
The usage isn't all that common in edited prose, apparently, and I can't find any stylebook advice or commentary on the issue; anyone else?