The cost of email archiving is comparatively cheap compared with the cost of mounting a legal defence when you have no evidence to protect yourself.
The use of email archiving from a management point of view is amazing, when you are looking after 120 sites over a 10 year period, all with differing issues and technologies, having an email trial of who said what, who did what and when is very useful.
When engineer X in 2009 made a change, we can look back and see why he recommended that change.
Is email archiving for everyone? Definitely not.
I had one incident with a company who had a 7 seven figure contract for a very long term project, retrieving those emails from 2012 meant that a huge amount of money was not lost. They now have an email archiving solution for such incidents.