I'm not sure on that last point
But really, it's peace of mind.. when I was running my 30 with stock rods, no balancing, a 6,800rpm rev limit and 250rwhp, the stock balancer was fine.
Now that I'm spending $_$ on building it properly and plan to beat up on it a fair bit aswell as make twice the power it was making before, a new quality balancer seems the way to go.
The major deciding factor for me is that after lots of research, it seems the majority of the oil pumps that die have stock balancers strapped to the front. Good balancing and a good harmonic balancer should go a long way to keeping your bottom end together.