Cluster: How To Use It?

Hello, everyone.
So, I've just reached level 99 on my archer (dxp yay), and went to my bank to get my Cluster, which I've been waiting to use for a while. I decided to go to my favorite grinding spot to try it out... and I'm honestly pretty disappointed. So my question is, what makes Cluster a level 99 legendary weapon?
-Could it be the Super Fast attack speed?
Well, I found this bow, which is a level 98 unique, with slightly less damage:

Why use a Cluster when there's this unique bow at a lower level, and at the cost of -regen and a bit of defense, has mana steal too, along with regen?

-Could it be the high poison damage?
There's also this, which I've had for some time:

It's max poison is about 600 dmg over Cluster's max poison, and it also has a larger amount of life steal. Hey look, it even has exploding too, and a bonus of loot bonus and +strength! (And it's a unique 10 levels below Cluster)

I'm sorry if I seemed really annoying by writing this, I'm just blind to what makes Cluster one of the endgame legendary bows, when it's really lacking compared to other Legendary items, Rares, and even Unique items around and below it's level.

 

I believe that cluster is strong with powders.
Also, can I buy that black arrow

 

Cluster has four slots for powder and no skill point requirements so it can be used in any build for a ton of elemental damage. High poison and exploding are also a good bonus.

 

I love Cluster! It has great powder slots, poison, exploding, mana, and I don't need to set any skill points to use it :D