[The Saucepan Man]

Yet Rivendell would have been a great prize for Sauron, and one which he would have taken, I think, had he been able. This is apparent from what Gandalf says in the Quest of Erebor in Unfinished Tales:

I thought then, and I am sure now, that to attack Lorien and Rivendell, as soon as he was strong enough, was his original plan. It would have been a much better plan for him, and much worse for us.

Indeed, Gandalf had in mind the protection of Rivendell in prompting the Quest of Erebor:

You may think that Rivendell was out of his reach, but I did not think so. The state of things in the North was very bad. The Kingdom under the Mountain and the strong Men of Dale were no more. To resist any force that Sauron might send to regain the northern passes in the mountains and the old lands of Angmar there were only the Dwarves of the Iron Mountains, and behind them lay a desolation and a Dragon. The Dragon Sauron might use with terrible effect. Often I said to myself: "I must find some way of dealing with Smaug ..."

So, in sending Bilbo off with Thorin and company, Gandalf was hoping for the re-establishment of the realms of Erebor and Dale to provide protection against an invasion of Eriador by Sauron through the northern passes, while eliminating a potential (and powerful) ally for Sauron.

Had the northern passes been open to him, I do not believe that Sauron would have hesitated to make a move on Eriador and, most particularly, Rivendell. So, why did he not do so when he had fought back the Dwarves of Erebor and the Men of Dale? Well, I think that he recognised that, given his committments in the South and elsewhere, his army in the North would have been cut off, and quite possibly eliminated, by the forces that he was at that stage besieging. So he chose to keep them penned in while he dealt with Gondor, believing that, once he was victorious there, he would have sufficient forces to then eliminate the resistance in Erebor and Dale and forge a way through the northern passes to Eriador.