How often do you have non-fused iterators?

I've been noticing some places where I could do iterator libraries slightly nicer if they're fused, at the cost of needing the extra .fuse() flag on things that aren't naturally fused.

It seems like an easy win to me, since I can't think of a time I knowingly used a non-fused iterator. But is there some domain I'm missing where non-fused iterators are more common?