First, I want to distinguish between Deflection and Damage Threshold. Deflection is what enemies attack, it's like AC in D&D. Deflection is derived from the character's class, level, and other bonuses from gear, spells, etc. Damage Threshold is what's used to reduce damage once you get hit and it comes mostly from the armor you wear, though heavier armor slows the character's actions (attacks, spells, etc.) down.
The advantage to wearing lighter armor in PoE is a faster attack speed, which can lead to great offensive potential. If you wield a single-handed weapon with no shield, you will gain an Accuracy bonus, but not a Deflection bonus. A dude with padded armor and a rapier will attack frequently and with higher accuracy, but he'll be relatively easy to hit and take more damage. If you want a great Deflection, wear a shield and accept standard Accuracy (or even reduced Accuracy for heavier shields). If you want to soak a lot of damage, wear heavier armor.
ok i see.... was hoping i could create a dual wielding character.... dat could fend off multi foes without having to hold a shield.... may be with a talent to help back dat up or sumtin lol..... u know kind of like drizzt do'urden in r.a salvatore books.... thanks for the info though