Now, by "last hit assistance" I mean the below 50% HP -> 6/8/10% bonus magic damage to minions. What is amusing is that this doesn't have a CD meaning that if there are 3 minions below 50% then he can apply the passive to all 3 of them.
Consider the following scenario to further elucidate the strength:
At 10 minutes melee minions have 585.5 HP so when they're below 292.75 Zed will about level 8 or 9 but let's round DOWN and say he's level 8. I looked up Master Zed player, Laceration, at a game where he farmed decently well and performed decently to try to be as unbiased as possible. At 10 minutes he has built Youmoos in this particular game and has additionally purchased a long sword for a total of 65 bonus AD.
> Level 8 Zed
Base AD: 82.6AD
Bonus AD: 65
Total AD: 147.6
Now, to calculate damage done to minions below 50% HP.
> Level 6 passive damage: 6% MAX HP
So: (585.5)(.06) = 35.13 magic damage
Finally, total damage to minions below 50% with items and passive:
> 35.13 + 147.6 = 182.74 damage
So a little over 62% of the damage to minions below 50% is produced by the passive. It would make sense if they intended this to be a little assistance for lower elo players to last hit under tower however for high elo players that typically don't have trouble last hitting this is simply a bonus, an example is how low elo players don't typically know how to push efficiently but high elo players can abuse this. Similarly to how low elo players don't know how to use Ryze properly but the higher in elo you go the more he's abused (and even more so in professional play). The difference is that Ryze doesn't have a "handicap" for low elo. By no means am I comparing the two champions themselves, I am simply comparing the fundamentals of their gameplay with respect to influence in elo.