The Defy is a performance machine, yes it's aimed at the endurance sector but it’s still designed with the same goals as Giant's pro-race TCR or aero optimised Propel. So low weight, stiffness and all of the characteristics Giant applies to pro-level bikes are key.
The Defy is also Giant’s bread-and-butter machine; it’s globally its best-selling road bike, so it’s perhaps the most important bike in its range in terms of development.