Deflection stacking, deflection stacking all the way. A character with very high Deflection enjoys many advantages. He lasts very long in combat, and being heavily outnumbered and surrounded doesn't bother him that much, because it doesn't make him easier to hit. Deflection also avoids many attacks altogether (Knocknown, poison, disease as secondary effects).
I don't read about DR stacking on this forum nearly as often. Don't you think DR stacking is too weak ? DR comes with more limits, for example at least 20% damage always goes through. Each attack always deals damage no matter how thick your armor. Unless it doesn't hit at all.
Games like DCSS have limits on similar mechanics, such as shield parry. Each subsequent attack has a higher chance to hit. In PoE terms, it could be that when you are attacked, you get -5 to Deflection for 1 second (STACKS).