I didn't say the item was worthless. I said that your change, an active of 5 seconds, would make the item useless. And for reference, and I don't mean this to be offensive, the term worthless means "without worth" meaning that the item is very bad. Since your argument seems to be stating that frozen mallet is good on everyone, it is the opposite of worthless.
Frozen Mallet is very powerful on the right champion. But those champions usually aren't adc's. FM is geared towards fighters and bruisers like Darius and Shyvana; champions that need to stick to their targets and usually have to expend resources getting to their targets.
Personally, I wouldn't really mind a Naut with FM. He has enough CC otherwise that I'm probably dead if I get caught by him regardless. Also, if he invested in just health and magic resist, he is kinda weak against physical damage at that point and perhaps I can just beat him in a fight.
Also, mallet is HORRIBLE on Vayne; since she has to be within most CC ranges/dashes to attack, her opponents can close on her regardless of her 30% slow. It is much more beneficial to pick up more lifesteal/banshees as an adc. You might be able to justify mallet on a long ranged carry like jinx or kog, but ehh.
Frozen Mallet gives a small amount of attack damage, some health, and the ability to stick to a target. That's it. It doesn't give any resistances (health as a tanky stat only goes so far) and it gives no initiation power. The ONLY champion I rush the item on is Shyvana because the resistances she receives via her passive makes it a slightly more cost efficient item. Other than that, this is mostly a late game item because the ability to slow a target down doesn't matter if you are dead; something frozen mallet doesn't really do a good job protecting from.
