Defence is not 1:1 with damage - it's 1:1 with ATTACK.
That means that if your opponent has 2000 atk and 3000 bonus damage (5k total), and you have 900 def, the base damage will be 4100 AND THEN masteries/resistances/wrath will apply.
And that means that , in your example, the damage recieved with 900 defence and 500 resistance would actually be 2050, not 1400.
But let's not forget that there are tons of variables. For example: how exactly does resistance work? Does it subtract from the opponent's mastery or from the damage output? And thus, if the opponent has 0 mastery, does that mean that 500 resistance will reduce damage by 50% or will it not have ANY effect at all.
Bottomline: Defence would be a good stat to gem-soccet/refine/id IF solarflares and ragefires, bonuses, base stats etc offered the 2 stats in a 1:1 ratio, or at least close (like with acc-eva) but since both atk and defence effects are buchered down a lot by wrath, and since def is very low compaired to atk+bonus dmg to begin with... It's not really worth it in PVP.
But in PVE there is no wrath effects. Moreover, instead of 1-2 players hitting for much there are many mobs hitting for little, so defence's effect is multiplied. Thus, it's an excelent PVE stat.
Because this makes perfect sense, tallies with my own experience, and I'm sick of people spouting stuff that they obviously don't understand, with an utter disregard for context.