Why didn't Sauron make use of the dragons?

[Hilde Bracegirdle]

Ran across this last night and thought it would be good to add it here.

Therefore he (Sauron) hid his knowledge of Saruman’s double dealing and concealed his wrath, biding his time, and preparing for the great war in which he planned to sweep all his enemies into the western sea. At length he resolved that no others would serve him in this case but his mightiest servants, the Ringwraiths, who had no will but his own, being each utterly subservient to the ring that had enslaved him, which Sauron held.

UT IV “The Hunt For the Ring”

Note that this was very early on just after Gollum was released.