So with the OEM suspension the spring rates and rebound/damping are soft enough to allow the rear suspension to transition smoothly over bumps and undulations in the road while under load. So you can keep the stock rear bar and everything works fine.
When you change to coil overs with higher spring rates and stiffer rebound/damping and keep the rear bar, it doesn't translate very well over the road at speed anymore. It sort of skips around and causes the rear end to snap.
So an easy way to combat that is to soften the rear bar to your comfort, but in general the bar gets disconnected or removed because even the stock bar introduces too much snap oversteer, which you then have to try and dial out with your coilovers. Which is why i've never mounted my Hotchkis rear sway bar to this day. If you do a little research on the forum here, you'll find similar feedback from other fairly experienced drivers.
It's easier to remove the snap oversteer and then control that rotation with refined adjustments to the coilovers by increasing the stiffness one click at a time.
To illustrate, this is what it looks like with coilovers set to full soft settings, hotchkis front bar installed and the OEM rear sway bar on.
In the end it really all comes down to preference, this was excessive rotation for me so I changed my setup accordingly.