Having said that, we could possibly have it enabled per layer (so you could for example enable 3D on just one layer, then all 3D content has to appear on that - all other layers are still 2D and are treated as underlay/overlays). But I think it's odd to add a checkbox that says "Enable 3D" when nothing in the editor actually uses that!
That would work like After Effects does, per "layer" z-plane enabling. When you do it in AE, it allows you to move objects back and forthwards, so why not allow for that too?