Since stats weights are approximatively this (depending on your stuff) :
3.78 Int
2.95 SP
2.36 Hit
1.98 Crit
1.36 Haste
1.34 Mastery
Why not gemming +320Expertise/Hit instead of 80Int +160Crit for example or 160Hit + 160Crit ??
Just asking cauz i dont get it.