It is difficult to track which Mobile Devices should continue to be allowed access to the users vault. This leads to security issues as the user doesn't know which devices should have access or not.
When logged into the lastpass.com website vault, going to Settings : Mobile Devices shows a list of mobile devices that had LastPass installed at some point. All devices are listed with a random number as the name unless the user went to the website to update it immediately upon using the mobile app for the first time. It seems like the only way to clear out old devices is to delete all devices and start over.
The Mobile app should have a way to show the device UID, perhaps in Settings: Account: About. Adding additional information to the Settings:Mobile Devices page such as Created date and Last Used date would also be extremely helpful in helping prune this list.