Comment by pornel

Yes. It is required for W3C's DOM APIs, which give access to parent nodes and allow all kinds of mutations whenever you want.

Event handlers + closures also create potentially complex situations you can't control, and you'll need a cycle-breaking GC to avoid leaking like IE6 did.

You can make a more restricted tree if you design your own APIs with immutability/ownership/locking, but that won't work for existing JS codebases.