So why do we have Silver Edge when you can always pick Viper? Having more options to deal with something is better than less. That makes you think of the trade-offs between each alternative and then force you to make a decision on which approach would be the most effective one. Why do we have MKB and Bloodthorn to go through evasion? We could always pick Ember Spirit with True Strike talent or simply use heroes who don't care about evasion at all. There's a reason why Slark was the only hero with the same tool at his disposal. And he pays a hefty price for it, despite the skill being on a much-higher cooldown, making the idea of trading other high-cooldown skills like song for it much more reasonable than in the case of willow. Simply having the skill grant something like 100% physical damage resistance + 50% magical damage resistance + spell reflect would still make it one of the best defensive tools out there, while also allowing for more interesting counterplay opportunities.