Why I cannot use tuple as application env key?

[Max Lapshin]

> The key is documented as being atom. Unfortunately why is not documented.
>

Yes, and there is a simple check in application_controller that
disallow anything except atom.

> {backend, [{Name, Configuration}]}

This will give me full proplist on each access.  I have tried it with
thousand of elements and found that GC is going mad on high traffic.