you can use Your V12 versions as long as they will run on Your hardware/OS. You just can't buy V12 anymore and there will be no support besides of helping Yourself (reading manuals, etc.)
FM12 is already no longer under developement - no more updates
End of Support does not equal End Of Life
Some of the reasons may also be technical or security related changes
The install notes for the latest Server 14.0.4a patch http://help.filemaker.com/app/answers/detail/a_id/15413/
With this patch applied, if you enable the Use SSL for database connections setting in Admin Console, clients using FileMaker Pro 12 or FileMaker Go 12 will no longer be able to connect to solutions hosted by your server. Follow these uninstall instructions if you need to uninstall the patch to allow FileMaker 12 clients to connect.