Jonnosaid:
This reminds me of that discussion between Lucas and Marquand regarding the rebel briefing room in RotJ - the one in which Lucas declared that the rebels were winning because they had the best equipment (can't remember which thread that was cited in - perhaps this one?)
There's no logical reason why the Empire, with their unlimited reach and resources, would purposely field an inferior fighter as an absolute standard - even taking the argument that the first-line flyers are disposable, how come we never see the big guns?
Worse, the idea that X-Wings are just better ships serves to further undermine the whole ethos of the rebellion. As far as the films present the situation (and that's where I personally draw the line), the X-Wing vs. TIE match is at the very least equal, and the outcome of a dogfight entirely depends on pilot skill and courage... and that's the whole point. Or at least it used to be.
Ok, this is from my X-wing vs TIE Fighter days many moons ago, but it's a good as explanation as any:
The X-wing is a multi-role craft, it has decent speed, guns and maneuverability and a hyperdrive. It was a secret design for the Empire, but the plans were stolen - at which point the Empire just thought 'fuck it, why bother it's out there now'. Before this, the rebels were using Z-95s as their primary superiority craft.
The stock TIE Fighter is short-range and weaker, yes. But it's just as fast as an X-wing and more maneuverable. They can also be mass-produced cheaply and quickly. That's why the Empire fields them. They aren't as over-engineered as the X-wing and that has positives and negatives, but they serve a specific purpose: which is air superiority and defence.
Now, by the time we get to ROTJ, the Empire have the TIE Interceptor, which is pretty much the MiG to the X-wing's F16 and far more dangerous to the X-wings in a 1 on 1 fight. It has the same gun array as the X-wing, is just as cheap and quick to produce as the original TIE, and still just as fast and maneuverable. If you're a hotrod Imperial pilot, this is the one you want to be in.