I believe it was for PVP purposes and the fact that sometimes even after several minutes has passed since the heavy drop, some people are still running around with machine gun ammo effectively using it as a primary.
The counter to this argument is that in the D2 beta, the fusion rifle gets six shots after picking up power ammo effectively making it the new "machine gun" of D2 so to speak.
Thoughts?
[i]~Nyahh! =^,.,^=[/i]
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Watch Machine guns will be introduced back into D2 with a paid expansion is what I'm betting