Wouldn't that depend upon your situation? As a former employee of an Education Service Center (an ESC) in Texas, which acquired ISO Certification, document/records retention policy (for 'legal'/other reasons) was 5 years.
If taking one's content from a 1.9 and migrating to a 2.x, yes, limited restore. One might only really interested in the content/activities/structure in/of a course so one doesn't have to 'start over' or re-invent the wheel not the students work/grades, etc..
If one were under guidelines such as mentioned, not only would one need to keep the 1.9.x backups, but also acquire an archive of the 1.9.x Moodle code/site and have the ability to restore should a serious legal question arise.
Before dumping all those 1.9 backups, check with your entities records/retention requirements/policy. Better safe than sorry! ;)
'spirit of sharing', Ken