A Ton of Questions

I have a few questions related to valuation. There a number of them, so bear with me:

1.)Unlevered Free Cash Flow #1: Why do we value a company based on unlevered free cash flows that yields an enterprise value? Why don't investment banks just make their lives easier and use levered free cash flow which would theoretically yield an equity value and eliminate unnecessary work?

2.) Unlevered Free Cash Flow #2: Why does unlevered free cash flow subtract taxes? I thought the whole point of valuation was to value a business's core operations? Isn't it sort of counter-intuitive to tax effect EBIT when countries pay different tax rates? How can you compare companies this way?

3.) Non-Cash Add Backs: Let's say that you have impairment charges of $100. Some source's have said not to add back the charges from the CFS because it is a non-core, non-recurring expense. Why is this so? I could understand this if you also did not include it as an expense in the NOPAT calculation, but it does not seem like the source I am working from is doing that either.

4.) Non-Cash Add Backs #2: When calculating UFCF, do you add back changes in deferred taxes? Some would argue that this should not be added back in for some reason, but I cannot figure out why it wouldn't be.

Any thoughts would be greatly appreciated. Have a happy fourth to all of my friends here in the the United States!