At least Stormtroopers look kind of scary and a few of them can aim.
Right before the Battle of Geonisys, Count Dooku tells Mace Windu that the Jedi are severely outnumbered, and in march a bunch of battle droids. If we learned anything from “The Phantom Menace,” battle droids are easy to kill and only exist in this context as an excuse to use special effects (which suck, even for 15 years ago).