This allows for configuration options that must accept arbitrary variables. Even if the node's value is an array, VariableNode will not perform any special processing/merging as is done for ArrayNode. It functionally behaves like a ScalarNode.
I had thought that this was unnecessary - when would you ever want to just let "extra" options fail silently?
But, the SecurityExtension takes advantage of this by creating two separate config trees. The first tree looks for just one particular value on the configuration array and ignores the rest. So, there *is* a use-case for allowing all extra fields to simply be ignored, though this should not be the norm.
We decided that this is not necessary and that it's leaving too many things wide open. Instead, in these cases where we have an array with unknown items, a prototype should be used.
Further functionality will need to be added later to allow you to specify a few nodes that you *do* know about under an array and then also specify a prototype to catch everything else.
This is *usually* what you want (and is defaulted this way). If you have an entry in an array *just* so it can become the key to that entry later, then you shouldn't normally still need it in the resulting array.
The importance of this comes in with validation. Since we're throwing an exception if you have any unrecognized options, the presence of the "key" field in the resulting array will cause issues when it's not needed.