It's not a question of backup and time required to backup, it's more of a question of what happens when your company gets sued. The plantiff's legal team could request that all of your records relevant to the complaint be turned over. It could take hundereds of man-hours to go through decades worth of data to find the relevent documents. After all, you don't want trade secrets and the like to be entered into any sort of public record.
This is especially important in the healthcare field. Hospitals and Doctors get sued all of the time. To respond to discovery requests and subpoenas, they can waste a lot of the IT team's time just scrubbing through years of personal e-mails. When I worked at major hospital a few years back, this was a huge concern.