NV had spears, they worked fine.
No they didn't, you couldn't stab with them, just throw them.
Spears are inherently balanced. It doesn't have the long range accuracy of a bow and are near useless in close quarter combat against traditional close quarter weapons. It excels at midrange combat and getting around shielded enemies without getting too close.
It comes down to animation difficulty, laziness, and time constraints. <_<
Why does everybody bring up the freaking animations? Of course it's relatively easy to do, that's not the problem.
You mentioned how they're near useless close quarters, I agree, that should be in the game, but I don't know how could it be made to work in a first-person game that you can't stab/slash somebody who's really right next to you, not to mention, how would you make it clear with the player when is he in reach.
Just thinking this over made me think of a really uncomfortable system, ironing these out would take way too much time and resources, they could be spending on something else...