I've used Cell::trace quite heavily in two web apps already, to keep things alive which may not be observe at the moment but may be observed later by a switchC etc.
It is sort of like Rust's Trace trait in that it gives sodium the ability to track sodium objects within the cell's value.
I think its a very useful primitive, i'm just having trouble convincing others. So it may just end up in a separate sodium utility library one day.