is there any reason to store session keys with empty user_data field?

We used to use database for sesssion keys and switched over to REDIS about a year ago. Works great, super fast but I realized a huge proportion of our database is userd up by session keys with empty user_data, these are most likely from bots and batch jobs but it seems really redundant to store them all. Are there any security reasons to store them all, perhaps to be able to investigate DOS attacks?

thanks

Will