From what I understand, EVA-01 and Rei were necessery for 3rd Impact. EVA-01
as a stand in for Adam, and Rei to control it, to whatever end. Without them
it wouldn't be possible.
So then, why do NERV continually put them both in dangerous situations? OK,
you can (presumably) clone a brand new Unit 1 and Rei (although don't they
need Rei's body intact?) if either get destroyed by a Angel attack, but
since they can build as many Evas as they like, and they know all the
potential pilots, why not keep EVA-01 and Rei out of harms way?