In many high speed/ high downforce style racecars with significant suspension travel (ie. more than 1" total) mechanical grip is hugely diminshed if the dampers do not have any high speed rebound force. Looking purely at the contact patch hysteresis a tire sees, using only HS compression damping can cause very large accelerations in the rebound direction (and very large forces in the compression direction) and the tire load variation becomes worse. There may not be potholes on most racing circuits (of course there are some tracks that pretty much do) but seemingly slight disturbances in the surface profile become very exagerrated when struck at 210mph. Tire vibrations can be damped in both the rebound and compression directions, but using only HS compression (or only HS rebound) will result in force spikes larger than if you had a reasonable distribution of both...and thus less overall damping force in each direction. A mass/spring/damper system vibrating would show a similar effect.
Excessive HS compression will also transmit larger forces from bumps to the sprung mass, which can upset the racecar as well as the driver.
I think that many open-wheel racecars depend wholly on "aero grip" and thus sprung mass control and tire alignment control are paramount. HS rebound can be unnecessary, in this case. As someone said, it is very application specific but HS rebound has more use in racing than just coming down off curbs.
I'm not sure I understand where 'high speed' rebound would come into play and therefore dont understand the need for 'high speed' rebound settings. Sales gimmick? Are 3 way dampers therefore sufficient?