Well it works well in LotR Risk!
Seriously, my thoughts were much the same as yours when I first read that passage in UT,
Lost One. But, when I considered it further, I concluded that (assuming that Erebor and Dale presented no obstacle), it would make sense for Sauron to launch an attack on Imladris via the northern passes. It is largely "friendly" territory. All of the other routes, although shorter in terms of distance, would provide major opposition for much of the way, whereas Sauron's forces would be unopposed all the way through the northern passes, into Eregion and on to Rivendell.
And I do think that the capture of Rivendell would represent a major feather in Sauron's cap. I see it as the seat of the White Council and the main base for anti-Sauron operations. So, symbolically, it would represent a major coup for him. Only Lothlorien and the Grey Havens were equivalent in importance, and (courtesy of Galadriel's protection) Lothlorien could not be vanquished without Sauron attending to it personally (something that he would no doubt be loathe to do), while Mithlond was even further away from Sauron's power base than Imladris. The capture of Rivendell would no doubt provide a base for operations against the Grey Havens.
So, on reflection, I tend to go with Gandalf on this one.
