But, why does this require a new machine with the accompanying huge (real, not tax) depreciation hit?  The same tax advantage is achieved by spending a like amount of 179a money on assets which someone else has already taken the (real) depreciation hit on, and you get several times as much real potential working capability for the money.  I do not get the cause and effect between the new machinery and the tax savings.  

Yes, I see how tax considerations reduce the after tax cost of a purchase, but no, I do not see how this has much of anything to do with deciding what to purchase.