The Darkwalker's shriek is not only useful to get a rid of enemies right below you, but can also force people out of Vipers and Mantas. Roadkill with a Darkwalker is impossible, so don't hope to crush someone by stepping on them. On top of that, some areas on maps are very hard to access using a Darkwalker. Crouching will deal with low overhangs, but the Darkwalker itself can easily "climb" some areas otherwise inaccessable.