I would try to use the spike traps over grenades. I think the damage percent is better. I also think the area of effect of the spike trap is better.
It'd difficult to tell not being in beta, but I think the fault of Spike Trap will prove to be loosely grouped enemies. If they are not closely packed, the trap springs on the first enemy, and then if there aren't a bunch of other guys around him, it only hits him.
The strength of Grenades is that I can use it to knockback the front ranks so they are packed more tightly, then they all get hit by Spike Trap.
Do you feel like you'll critically need bola shot aoe when you have the craziness that is the crimson cluster arrow and grenades?
If the mobs are positioned such that I don't need grenades and I already have my traps out, and I'm out of hatred, then yeah. But it's not critical. I made another build where my only hate builders are Spike Trap and Grenade; feels kinda funny not to have a shot that gens hate but it should work:http://us.battle.net/d3/en/calculator/demon-hunter#hflSTi!gUW!ZcZcbc
A few things to note: This build is optimized for 2h crossbows which are slow and have high avg damage. The hatred generators, Grenades and Spike Trap, do not depend on weapon speed from what I have heard, but do a lot of damage and generate decent hate. This makes them perfect for use with slow weps.
I see Hungering Arrow and Bolas as good hate generators for fast weps because you can likely get two of them off in the time you can get a Grenade or Spike Trap off; this is because attack speed is faster than cast speed. I suspect this is why Grenades and Trap generate more hate per use than the shots.
Bolas are replaced with Rain of Vengeance; a CD for difficult situations that is extremely hate efficient. My disc skills are geared towards hatred generation to fuel more Multishots.
I replaced Caltrops with Smoke Screen since slow weps greatly benefit from hate which does not come from using abilities. I may end up with Caltrops depending on how often I feel I can get enemies on top of it.
If I don't use Caltrops, I don't benefit from the Cull the Weak passive so I replaced it with Sharpshooter. I usually think of Sharpshooter as being fairly useless; usually you are going to be shooting so many arrows that even with a low crit rate, you will crit frequently so the benefit I projected was 3-6% crit max. However, it is possible non-arrow attacks like Spike Trap and Grenades do not cause the Sharpshooter timer to reset; in this case I could get a good period of time to build up Sharpshooter for a big MS.