Originally Posted by
Theory
As a support main, apart from the reasons that TyZi gave, I rush Solari when there's an assasin with lethality (duskblade to be exact) or any threat for my adc, especially since redemption takes around 3 seconds to cast when solari is instant,that 3 seconds is enough for the assasin to take down my ad carry without any problem.
This is the primary reason you should be getting Locket over Redemption. Sure Locket has a lower cd and is stronger with how much "health" it provides, but it's really about the role the item is designed to fill.
Redemption is designed to fill a rallying effect, an item that extends your team's ability to fight over an extended period. It's not for on reaction, but it's generally more versatile in either mid-team fight swings or temporary boosts to extend a siege. Locket is designed as a stopgap against getting burst down. This is on reaction to a big piece of damage coming your way, and is highly dependent on timing. Even against non-assassins, Locket can be invaluable for mitigating otherwise catastrophic amounts of team damage, which ults like Ori or Gnar can perform. Mitigating maybe a third or a half of all damage those ults deal can be the difference between outright losing a fight and winning by a hair. That's where the instant activation and higher health pool really shines. Even just activating once the teamfight really starts can mitigate more damage than Redemption would heal or deal combined.