I feel diversity amongst the classes is important, it's what makes each class feel more unique to the next.
I'm not against the concept of all three classes having the freedom to use any weapon but perhaps how they handle certain weapons could add to that diversity.
[b]Hunter[/b]: Slightly faster rate of fire on Scout Rifles (.1-.2 seconds?)
[b]Warlock[/b]: Less recoil with Handcannons
[b]Titan[/b]: Less recoil with 3-burst rifles
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The upgrade systems associated with uncommon-and-higher weapons and armor will overwhelm any class-or-race specific bonuses.