So why did cutting the bridges foil Smaug's attack on Laketown in the book the Hobbit?
I always thought it was due to the fact that the houses were too small and flimsy to land on, meaning that he could only try to flame them. And in flying, the updrafts/downdrafts over the burning town surrounded by a cooler lake would interfere with his flying, and the smoke from the burning houses would obscure his aim.
and of course, it was a moonless night, so figuring out how high he was flying could lead to "controlled flight into terrain"...
Michael Martinez' take is here.
but since xenite won't load here, I'll have to keep guessing.