Although Bob's comments are refer to physical tapes (and are correct) they also apply to 'virtual' tapes. Obviously, the chances of a virtual tape being faulty/damaged is lower than that of a physical medium due to the lack of 'mechanics' and the poor status is almost certainly due to an IO failure during a backup. To check this, identify the sessionID of a backup on some of these poor tapes and check that session for IO failures. If this seems to be the cause of your virtual tapes becoming poor then it's a fairly safe bet that you can use reset_poor_medium.
However, because virtual tapes are deleted once their contents have expired and are not reused (like physical tapes are) there seems little to worry about.
Hope this helps to explain things.
John