Hierarchical layers
The possibility to group layers adds a great bit of flexibility and can help to maintain a clean structure. For instance, when working on a design that encloses two rooms, it would be nice to have those rooms modeled as parent layers with all the different structures as child layers. This functionality can be seen in almost any CG software, e.g. Photoshop.
Not currently planned.
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Sebastian Haas commented
I totally get your point there, however I guess this is a feature that could be added without affecting the current behavior at all. For instance, making layers a drop target during the layer rearrange operation. I'm pretty sure that users benefiting from hiding/locking/rearranging features are capable of handling the concept of a layer in a layer.
Just think of more than one room, more than one level, more than one table etc. on which you want to divide your tracks into different sections.