When is idempotency in API design really needed, anyway?

hey folks, i've been thinking about api design lately, specifically around idempotency. we all know it's good practice, especially for operations that modify state (like post or put requests). but how crucial is it really in every single scenario? for internal microservices where network flakes are less common, or for read-heavy apis, is the added complexity of implementing idempotency keys worth it? or should we aim for it everywhere to avoid any potential surprises down the line? what's your take on when idempotency becomes a must-have versus a nice-to-have