Without having thought about it much, wouldn't the Ayn Rand example show that the cases where the comma is necessary for clarity are exceptional, and not the norm? Shouldn't the comma, then, be used only where it is necessary for clarity, and not in other cases? Couldn't educated writers be trusted to apply the comma on an "as-needed" basis, much the way that all writing involves making choices "in the moment" in order to enhance clarity in ways that wouldn't have been obvious in the abstract?

I can see at least one argument for the other side: Prescriptive rules are easy, unthinking ways to protect against potential ambiguity. For example, I didn't recognize dangling modifiers until my ears were trained to listen for them, probably in my teens (is that what you call them? A sentence like "While hanging from a tree, the banana Peter was holding dropped from his hands" - you grammar mavens will know the correct term for this incorrect construction). Oftentimes, there's no ambiguity, because it's obvious from context who is doing the action (and I'm pretty sure there are dangling constructions that don't sound as awkward as the one I wrote, though I couldn't think of any on the spot). But sometimes, there will be ambiguity, and unthinkingly applying an across-the-board rule helps protect against those times.

Before anyone boxes me in as an "anything goes" grammarian, I'm actually a staunch supporter of the comma. Also, I very much appreciate learning what it was called, through this thread. Thanks, Amanda and Thalia!

-- MichaelBerkovits - 23 May 2008