The rationale seems to be that even if your rate of climb at Vyse is pitiful, with no terrain to worry about it may be better to accept even half of pitiful in return for the better control that a speed above Vyse offers, since fatalities are typically associated with loss of control rather than CFIT. Since the curve is relatively flat, you may manage quite a speed increment without losing all your climb capability.
I can't see the point in step climbing. Surely a sustained, trimmed, shallow climb is preferable, and will return the same or better average RoC without having to flirt with Vyse from time to time?