Improve feature flags

feature-flags.md

@@ -17,7 +17,31 @@ Why use feature flags?
   failure.
 * **Gradual rollout**: feature flags enable gradual rollout, where a piece of
   code is gradually activated, for instance on a per city basis, or on a per
-  user group basis. This builds confidence in the feature release process.
+  user group basis. This builds confidence in the feature release process, and
+  allows the engineer to verify that the new implementation is actually better
+  (for instance, when coupled with A/B testing frameworks).
+
+Should feature flags be used for everything?
+--------------------------------------------
+
+I don't think so. I think it's a matter of good judgment. Just like 100% test
+coverage does not always make sense (provided lines that are not tested are
+explicitly ignored), adding or not adding a feature flag is an engineering
+decision.
+
+When would I not use a feature flag?
+
+* Simple changes: copy, logging, etc.
+* When rolling back takes a few seconds
+* Feature that is used only in asynchronous jobs that are safe to retry and
+  don't impact the user experience.
+
+When should a feature flag be used?
+
+* Large refactors
+* Changing integration points
+* Performance optimization
+* New flows
 
 References
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