Hello knight81,
virus definitions are like dictionary. It used for knowledge for virus. Therefore if your sep clients have latest virus definitions can find virus But as you well know every new version fixed some bugs, some trics for do it things fast.
Everytime new version is good (if checked), I remember one torjan infected my clients machine and everytime show pop up windows as like "mail cannot send this recipient" I fix this problem with update my sep clients. This is only example.
Best Regards.
Fatih
Ps: 10 can detect firus ofcourse if have latest definitions:)