Pauses

Straight reference counting works if you can tell who the 'users' are, and if the data structures are fairly simple. But we can't really ask everyone touching a graph to walk the entire graph and all its data structures and increment, then decrement, all its reference counts (that would be much more cache-unfriendly and unpleasant to maintain than what is done now); and because nodes are often shared between graphs we can't have one giant reference count across the whole graph... and if the reference-count-freer walks the graph and only frees if all connected entities are at refcount zero, you might as well drop the reference count, because what you've got there is a garbage collector. :)