Email Merge never interacts directly with your system address book. However new versions of Apple Mail, while they are processing messages created by Email Merge, can behave in a way that causes your address book to be wiped. We apologize for this and addressed it as soon as we became aware of it - Email Merge X 2.3.6 fixes the problem.
A more detailed explanation follows. Apple Mail, the Address Book application and Safari all access the system address book file. The file is located in [your home folder] -> Library -> Application Support -> AddressBook. Since more than one of these applications might try to access this file at the same time, there is a synchronization mechanism to ensure that they don't interfere with each other. When one application tries to access the file, it first sends a message to the other applications (if they are running) to check they are not accessing it.
After Email Merge has sent Mail a long list of commands to create some email messages, Mail spends some time processing these commands. This processing keeps Mail very busy and, as a result, unresponsive both to the user and to messages from other applications. When Address Book asks Mail whether it can access the address book file, Mail takes too long to respond. It seems that Address Book concludes that there is something wrong with the address book file, and so it starts a new one from scratch.
This problem appeared in recent versions of Apple Mail, and seems to be more prevalent on Intel-based Macs. To be frank, this is a poor implementation on the part of Apple's developers. Failing to get a response from another application should never cause data loss. Nonetheless it is of course unsatisfactory for this problem to be indirectly triggered by Email Merge, and we apologize for any inconvenience caused to our users.
In order to fix the problem, Email Merge 2.3.6 changes the way it communicates with Mail. Previous versions would send a long list of commands to Mail, and then let Mail process them in its own time. Version 2.3.6 sends the commands for one email message at a time, then it waits for Mail to finish processing, before sending the commands for the next message. This prevents Mail becoming busy and unresponsive, and gives it a chance to respond to requests from the other programs accessing the address book file.