There are three exceptions people sometimes make in their avoidance: Using non-free drivers, using non-free software for work/school, and using non-free software in the browser. It’s rational that someone would want to limit these exceptions as best they can, and LibreJS is useful for that.
I use LibreJS not to help me completely avoid non-free JS, but to make me more concious of my software use. If a site has non-free code and LibreJS pings me about it, I’ll consider if there’s a better site I should be supporting instead.