As to why there are no Battle Droids/ Droidekas in the Original Trilogy (besides the fact that they weren't a twinkle in Lucas's eye yet, or were they? Only he knows.)
He doesn't seem to be a Droidist since the Empire uses other droids. Probe Droids, The Death Star Droids, Astromech Droids, Interogation Droids, IG-88, etc.
The Droidekas seem to be very effective. (seeing as how not even a Jedi can take one on) Exspecially with thier sheilds, except when hit with Starfighter lasers. Though I'm sure thier sheilds could be upgraded against that.