Mostly is because:
Either yes or no, please state your reasons. Thank you. 
<3
Drakoniak2 votesNo. Reason: High cost for small profit, Instead you could buy Raduin which is cheaper and much more valuable due to the armor it provides +passive and the active that allows you to stick to the target.
Frozen mallet is in a bad state atm the item needs severe changes, just because it gives a lot of hp and slows the enemy with AA´s it doenst justifie his gold cost.
BeatzBoyFTW1 votesNo. Reason: High cost for small profit, Instead you could buy Raduin which is cheaper and much more valuable due to the armor it provides +passive and the active that allows you to stick to the target.
Frozen mallet is in a bad state atm the item needs severe changes, just because it gives a lot of hp and slows the enemy with AA´s it doenst justifie his gold cost.
Well actually, it's fine not being a primary item. Usually I recommend this item for the sake of Sion Q complainers as mentioned :p But I can definitely see the item strength & costs difference you mention, makes more sense.
k wìx1 votesI can see it - Where as Randuins is a one-trick pony item that the enemy team can bait out, Frozen Mallet is practically an aura - Score one auto and you should have a much easier time landing a full strength Q.
Situationally, it does work.
Does it proc while he's using his passive and punching people to death? If so then thats pretty compelling too!
iainB851 votesThe problem with frozen mallet is you are sacrificing stats per gold for that passive effect, so unless you are ahead of everyone item wise, it actually puts you behind in terms of raw stat power.
Champion Skin1 votesFratmas 
Atmogs 
Well it was good before... not so much now...
AtheosisX1 votesI would suggest TF with it's movement buff over Frozen Mallet given a choice. Honestly though Sion is not built to chase, he's built to team fight.