Well, if anyone cares about the EU explanation for Tie fighters, I believe they switched to them (from whatever they were using in the clone wars) as a political favor to an ass kissing moff who had no tactical sense at all, but knew how to keep systems in line. The Emp didn't give a shit because he believed all he needed was the dark side and some pawns to rule. Later versions were supposed to have shields and hyper drives (Vader's craft was one of the prototypes) but the empire fell before they went into production. Not to mention, in 99% of engagements, the TIE fighters were perfectly fine, as no one but the rebels ever put up a real fight. TIEs ripped through planetary defense forces like a hot vibroknife through bantha butter. Oh yeah, and I think whoever designed the X wings for the rebellion was basically a genius, and that's why the Xwings were so advanced.