I don't know that I would agree with this statement, curved shapes are not necessary less STABLE, it depends on your configuration. While you don't want every member in your design to be curved (mainly due to cost of manufacturing and human error in forming), in some applications a curved tube can actually increase strength. Think about this, the strongest structure out there is an arch. If you have transverse loading, then a bent tube IS better, because some of the load is transferred into an axial loading. If you have pure axial loading, then a straight member is better, because it wont be as likely to buckle.