This has been going on for too long. Some of us, players, like to play this game strategically to the bone.
Disclosing the extra damage percentage would be very helpful to clarify, calculate, weigh and make sense of a hero’s special. It can even increase with the level of special skill too. From my personal point of view, there seems to be no sensible reason to disclose the percentages of almost every ability and keep this one under the hood.