Well if you've got two trees and two bushes, and you know one of them is destroying your sewer line but you don't know which one, then you might not want to kill them all just to be sure.

Or in some cases you might not even need to kill the whole plant, just dig a targeted hole and hack off the problematic root. It'll probably take years for a different root to find the same crack in your sewer line and start wedging it open.