If someone going the speed limit or slightly under the limit renders an express lane worthless–then is the main roadway really all that backed up to begin with? Which is to say, if the main roadway is so backed up that the express lane would save you time, then generally it doesn't matter if there's a slowpoke in the express lane: it's still saving you time.
The issue I saw in Atlanta (and bear in mind, I
never paid for toll lanes) was that traffic would be slowed down to, let's say, 35mph-or-so in the general lanes. The express lane would be clear with a speed limit of 55mph to 65mph, but some idiot in the express lane would be going right along with the slow traffic, backing up the lane behind them. This caused drivers to illegally cross the double-white line to pass him, creating even more congestion and a safety hazard in the regular travel lanes.
While reversible double lanes won't stop some idiot from driving slowly on the interstate, it does offer the opportunity to safely pass the offending vehicle.
The main reason for building single express lanes on I-85 in Atlanta was that it was (comparatively) inexpensive to convert the existing HOV lanes, which suffered the same problem, into toll lanes. US-169 in Kansas has no such existing infrastructure, and most of the length where it is a 4-lane highway on this project could easily fit reversible lanes in the existing median.