[Lord Elrond]

I would also like to add that Sauron was not often the most patient being in middle earth. He oftentime overlooked things and did things too early and was too presumptious. He did have the strength too conquer much of middle earth. He wanted to cover all the lands with a second darkness and wether or not he thought he could do it at once or not didn't matter. He knew he could do it eventually. As seen in other battles the dark powers often attacked many places at once sometimes just on a whim or at a gamble that they might do harm. His true forces were where he wanted them. Orcs were of little value to him and were easily replaceable. I don't think having more forces attack Minas Tirith would have done much. It was primarily as pointed out the arrival of the Rohrohhim, Aragorn, and the death of the witchking that did the most damage and that was more mentally affective than just the physical numbers. Sauron didn't need more orcs at Minas Tirith so he might as well keep the other kingdoms busy and take a gamble at conquering or at least weakening them.