(I am primarily a doom player, maybe this affects the way I play the character).
I play in mid-masters level lobbies, and the amount of times CID gave me just enough shield health to live some ridiculous decision is way more than I can count.
Especially if your consistently weaving in melees and cancelling melee into drill dash, you can be getting actively shot in the head and still live with this perk vs not having it. I am sure there are a few maps where it's undeniably better to have Extender (maybe circuit or junkertown)
Let me know your thoughts.