We have tried the import without author emails (because that data is not available for older journals) in earlier versions of OJS (2.4.6), but there were error messages, coming from the database, if I remember correctly.
Also, if you will ever edit an imported article’s metadata, the edit form will keep showing an error when trying to save your edit until you enter an email address.
So - without diving too deep into detail of the OJS source code, we decided it was better in a long term perspective, to insert dummy mail addresses in the import XML rather than worry about an inconsistent database later on, for example when upgrading to another version.