Step-Throughs: an Advantage in Traffic?

There was an interesting post on ecovelo yesterday, where the author compared how he uses his three transportation bikes: a Rivendell Sam Hillborne, a Surly Long Haul Trucker, and a Civia Loring. All three bicycles are set up with upright handlebars and decent load carrying capacity, but the Civia differs from the other two in that it has a fairly low stepover. And according to the author, the combination of its "upright riding position, step-through frame, and internal gear hub make[s] the Loring exceptionally confidence inspiring for riding slowly when in close proximity to pedestrians and automobile traffic." Initially, I read past that sentence with the kind of matter-of-fact acceptance that goes with processing what you already consider to be a given. But then later I mentally "rewound" and thought "Wait a minute, he is saying that he finds it more comfortable to ride a step-through in traffic than a diamond frame - Is this a generally accepted notion?" 

I used to think the reason I prefer step-throughs for transportation, is that I often wear skirts. But having read the ecovelo post, I realise that even when wearing trousers I feel better on a step-through in traffic. And, assuming that Alan of ecovelo mostly wears trousers, for him there must be other factors involved as well. Maybe for me it's the promise of the easy "hop off sideways" dismount should I need to bail, that makes me feel more secure. But to tell the truth, I am not sure what it is, and whether my preference is entirely logical.

All factors remaining equal (upright handlebars, ride quality, load capacity), what, if anything, would make a step-through bike an advantage in traffic?