I've been in orgs in which developers are considered grunts and no one listens to their feedback. It's not that they aren't important, it's that the PMs/managers/whatever know better.

I see firebreak sprints as a symptom of an org not valuing engineer feedback. "Let the grunts blow off some steam or they'll get disgruntled and leave."

because sprints don’t allow the idle time that’s often needed to come up with such ideas. I could probably work for a year on “oh I’ve been meaning to look into that” tasks that have no clear outcome