Does the em-dash replace a subordinating comma?

If it were not for em-dash, we would no doubt keep the subordinating comma:

"It was only to be expected that Troy should fall — and fall emphatically, its very location effaced and, for millennia, forgotten —, because the Gods and heroes on the Greek side were among the most powerful in the Greek pantheon."

— modified from Understanding appositives and the use of the m-dash ( — )

ps thanks to Dinesh Kumar, for shortening the title. However, I don't see which 40 characters were added to the body. Can you assist?

pps other threads along this line have not resolved this particular point about using both em-dashes and the subordinating single comma. I retain this thread, because it is an example that cannot be fixed by simply moving the subordinate clause, or by dropping punctuation.

Solution 1:

It is a question of style:

Em dashes in place of commas

A pair of em dashes can be used in place of commas to enhance readability. Note, however, that dashes are always more emphatic than commas.

• And yet, when the car was finally delivered—nearly three months after it was ordered—she decided she no longer wanted it, leaving the dealer with an oddly equipped car that would be difficult to sell.

From the Gramnarist;

• Em dashes set apart parenthetical phrases or clauses in a sentence. In this use, em dashes are similar to commas and parentheses, but there are subtle differences.

• For example, em dashes are used when a parenthetical remark contains an internal comma or would otherwise sound awkward if enclosed by commas.

• Perhaps a useful way to think of the em dash is as a pause or parenthesis with somewhat more emphasis than a comma and somewhat less than parentheses. Here are a few examples of em dashes used well for this purpose:

  • Steely Dan’s title track to FM—a justly forgotten, Robert Altman-inspired 1978 comedy that tries to pass off Foreigner, Foghat, and REO Speedwagon as paragons of rock rebellion—initially sounds like an extension of that movie’s middle-of-the-road sounds. [AV Club].