This question was asked to me by someone while I was at an after-party for BlizzCon. I felt it bared repeating here so that others could find the answer too.
Question: Why is it that with some models, I can be selecting gear where the order doesn't really make any sense then suddenly it all gets sorted out like it should be?
The Short Answer: Because some models have their DPS/whatever structure tied to having an appropriate weapon equipped.
The Long Answer:
The Hunter model has been designed such that all DPS Calculations are invalidated to 0.00 DPS when you do not have a Ranged Weapon equipped. Why? Because if you aren't shooting something... you aren't doing any damage.
Why not leave Pet DPS to do damage to even without it equipped? For simplicity, Pet DPS is factored heavily by things that are done with Hunter shot rotations and leaving it at zero too prevents me from having to insert several fail-safes I would otherwise
have to implement (which cost performance when I don't need them).
DPSWarr is also set up this way, you need to have a Main Hand Weapon equipped before any item will show a DPS value. Fury Warriors do not require an offhand to get DPS Calcs, just the mainhand, though it could be argued that I should make it work that way.
Most other models do not need this practice in effect, such as Mage or ShadowPriest, because your DPS isn't physically dependent on having a weapon equipped.
If I were the owner of models where this would make sense, such as Enhance or ProtWarr, I would probably put these things in them too, but that decision is left up to their respective owners and I wouldn't require them to make this change.