In general, you don't really need faregates. They're obstructive and maintenance-intensive, and you can more easily enforce fares via POP.

When you get to really high crowding levels that equation changes, but BART doesn't have those crowding levels, it's not Paris or New York.

BART sometimes uses faregates for crowd control at downtown SF stations by temporarily disallowing entrances

I thought they just did that by disabling the down escalators (downtown BART is beyond thunderdome)

I'm tempted to go gaze at Embarcadero and Montgomery at 8:30 next time I visit. Their daily ridership levels don't scream "zoo."

Disabling one of the down escalators is (or was) a regular certain-times-of-the-day (pm rush) thing, but closing the faregates is more of an emergency thing. (Disabling the down escalators seems because people don't walk away from the escalators when they get to the bottom.)

Do you know what makes it easier to add more vertical circulation if that's your only capacity problem? Not having faregates. #sorry

I think the problem is platform crowding by people waiting for east-bay-bound trains (where each line is running every 15 minutes)... due to people not moving away from the escalator, leading to the escalator dumping people into an area that can become dangerously overcrowded.

So the issue is that in the afternoon peak, people loiter on the platform waiting for their line?

Yes, at least in 2016/2017. I stopped commuting through Embarcadero in December 2017. And I wouldn't say loiter -- there's crowding (and lots of lining up) waiting for east bay lines on the platform (and that's where you're supposed to wait for trains!) during the pm rush.