Does Rakinishu's Blade have a special purpose?
Solution 1:
It is used for an achievement and in the other hand it is Easter egg.
Achievement: "Bashanishu - Hurt Bashiok using Rakanishu’s Blade."
Rakanishu's blade is a easter egg from diablo 2 monster Rakanishu.
Bashiok is a monster created after an blizzard employee Bashiok.
Solution 2:
There is no known use of it yet, other than earning the Bashinishu achievement.
For this achievement, simply hit Bashiok with Rakinishu's Blade.
There is a banner accent as a reward for this achievement.