im sure they budgeted out primary stats for the gem slots. i think the problem would be they budgeted the items with green, or possibly even blue quality gems in mind - i mean, remember, that these are hellfire greens/blues and not intended to be used at endgame 70 and have epic gems. and especially not made with the thought they'd be used with epic gems against lower lvl players w/o them. they were made with the intention of getting i, toss some cheap green/blue gems in there, and replace it in a a few levels, tops. bit of a miscalculation by blizz, but thats not really anything new
at least thats my theory on the budget of items w/ gems