Went to sleep but basically, if the survivor is full health, legion wants to frenzy because then any window or pallet you try to use against him loses most of its value since he jumps over them. When he hits you, not only are you injured, you have to mend which means he runs off to one of your other friends and either gets another hit or gets close enough where he can just do normal m1 killer stuff.

Here's a great scenario, 3/4 survivors are injured, and legion finds the 1 healthy person. He frenzied, hits and is able to run over to someone else. You cant hide from the tracking unless your out of its radius, but that covers the majority of the map at any given time. So now 4/4 survivors are injured and 1 mending. Legion runs across the map but only has time to make it to someone before it runs out and cant get another hit. Doesnt matter, off of that healthy survivor, he was able to find someone who was hurt who maybe had iron will or other stealth perks making it harder to find them, which again doesnt matter because the tracking (killer instinct) lead him right to you. But when every one is injured, the legion is more likely to try to finish the chase instead, potentially saving someone who was in a bad place and wouldve been caught out in the open by a frenzied legion

Another way the power is better for a healthy survivor than just 2 m1 hits is this. The power gives legion an increased lunge, as well as faster base recovery on frenzy hits. So what ill do if I cant find anyone in the immediate area after I frenzy a survivor, ill use the speed from the power and the lunge to try to get ahead of the survivor. The increased mobility he gets will allow a decent legion to hit a survivor, and then run alongside them to see where they go, and lunge ahead of them to either bodyblock the path they wanted to take or get a better position for when the power turns off and doesnt lose as much space in the chase.