You can spray an area with 15 or 20 casts using a moving bait, or even a worm and then move on. Have you thoroughly fished that water? It depends. I fish quite a bit of water with trees and brush clumped together. I see the occasional angler cover an area with casts and move on without placing a lure directly in the patches of trees and brush, especially in really deep water. They will fish up next to it, but often not in the middle of it. At certain times of the year, and under certain conditions that is where the bigger fish will more likely be. If you don't check those spots, you are missing a lot of fish. And I find it useful to use enough lures to cover the different water layers as @WRB stated in his post above as well.