Well, maybe Tolkien hadn't thought of anything so complicated back when he wrote the hobbit. But if you want a more complicated ME explanation, lets say:
Who has proven 'spiritual form'?
- Glorfindel, a great elven lord
- Nazguls, wraiths of mortal men
They both are connected to the ring, Glorfindel because he is a Noldo. Smaug is not connected to the ring in any ways.
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Wistful, willful, wingless, fly!
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