Canadiens said that it was purely to make them weaker, it was I that said that cost was an issue, which is supported as so:

*While highly maneuverable, the TIE/ln had no combat shields. It did not carry missile tubes, but such weapons could be added on if necessary. Life-support systems were not usually included, but each TIE pilot had a fully-sealed flight suit superior to their Rebel counterparts. The absence of a hyperdrive rendered the fighter totally dependent on carrier ships when deployed in enemy systems. Because of all of these weaknesses, TIEs were designed to attack in large numbers, overwhelming the enemy craft. The Imperials used so many that they came to be considered symbols of the Empire and its might. They were also very cheap to produce.

Despite the aforementioned design features, TIE/ln laser cannons were relatively strong, and a direct hit on a starfighter or medium transport could damage or destroy it.

Contrary to popular belief, the ships did possess ejection seats, but the nature of space warfare often resulted in pilots riding their craft down to a swift end.

The relative weakness of the TIE/ln could arguably be explained by Imperial military philosophy, which viewed the starfighters and their pilots as expendable. Though Imperial pilots were of an elite stock, they considered themselves expendable, in accordance with their ideological training. Like stormtroopers, TIE pilots were addressed by a number. This procedure reduced them to being no more than anonymous and standardized operatives of the Imperial war machine.

*Courtesy of Wookieepedia.com