But this is another counter-argument to the evil Tyrael; not really I guess. Let me explain:
Wouldn't it be very very risky for the forces of Hell to allow Nephalem to increase in power? If majority of nephalem don't line up with Hell, they will have a worse fight on their hands. Granted we see weak willed humans able to fall to Hell's powers, but I still think it is risky.
Also, has this been talked about? I pulled it from wiki (so a pretend source):
The Pandemonium Fortress is the structure residing over The Worldstone has been held by both sides during The Eternal Conflict which so its architecture is "Shaped by Both of Their Cultures"
Thought Mount Arreat was the fortress guarding the stone... Retconned?