Defederations and bans essentially work like a shared blocklist for all users on that instance. Plus you get a local feed with other users who have similar interests, and blocked users/instances don't use local server resources.

Subscribing to a blocklist seems risky, because it's likely that many users would subscribe to a single, popular blocklist. Then it would become a centralised point of control which can silence people across much of the Lemmyverse.