It takes too long to get its effects, doesn't last long if you don't stack it, doesn't help her do her job at all and it's just boring
HP5 for 6 seconds is a tank passive, not a carry one. It's probably the worst passive in the game right now.
Hrm, that's all not really true.
If you're trading efficiently it's pretty easy to keep it stacked and it gives her quite a large amount of heal regen, which is very important in the top lane. It actually puts her sustain in line with someone like Shyvana by just hitting creeps, and blows Shyvana out of the water for sustain if you're trading with her and keeping your stacks up.
It helps her do her job quite a bit. in the top lane you need sustain. The top laners with poor health regen have shields (riven) and heals (rengar) for their sustain. Same in mid lane.
the problem with it is how it scales into late game, i.e. it's extremely sub-optimal, despite how the numbers work. it has a very limited time frame of usefulness, which is really bad. probably the shortest in the game, except for maybe Ashe's.
It's also just completely invisible power which is bad for gameplay readability purposes. You don't feel like it's giving you much, although the numbers say otherwise. That's bad for your opponent and you. Think the opposite of Draven bleed.
anyway, just food for thought