I don't know if this is the intend of Tom, but I was also stumbling on this issue and the only reasonable explanation I can come up with is that:

One key can't have multiple values with different types. For example in json:

  { "name" : 
    { "first" : "Bob",
      "last" : "Smith"
    }
  }

How would you add a "alternative" key to the "first" value type. The only way i can think of is something like this.

  { "name" : 
    { "first" : "Bob",
      "first.alternative": "Robert",
      "last" : "Smith"
    }
  }

or as @latk suggested, let "first" be an array, with it's first entry being the non alternatives.

  { "name" : 
    { "first" : ["Bob", "Robert"],
      "last" : "Smith"
    }
  }

I think this is by design. To quote @mojombo "simple configuration file that maps unambiguously to a hash table".

But this is only a guess.