During the process of trying to setup and test CardDAV synchronization of TB contacts (i.e., address book), which resulted in this topic, I learned something new about address books. When importing a TB address that has been exported some data contained within the address book that has been exported is missing from the one to which it has been imported. It is also missing from the file created by export which suggests that export is failing to function correctly. To early to say about how well the import might be working.
More specifically, some email addresses that may be present in the address book that is exported are lost by the export operation. Insofar, as there are lots of data items that contain NO data in my contacts it is NOT possible to say whether or not the same phenomenon applies to other data elements. It seems that export to .csv format is slightly better than export to to .vcf format in this case. In the case of .csv format contacts with more than one email address end up with 2 email addresses in the exported file. If there are more than 2 email addresses they are lost. In the case of .vcf format only one email address appears in the exported file no matter how many are associated with the contact in the address book that is exported.
It does occur to me that this finding may have something to do with the synchronization problem described in the referenced post.