No, you can't just copy default.php over. First the filename it self needs to be changed. For example if your template is the example template it'd become example.php. Next inside of the PHP file the classes have the template name in them for example "CBProfileView_html_default", which would become "CBProfileView_html_example".
I highly highly suggest leaving the PHP file alone. Anytime we make fixes to it you will not have those fixes and will likely be posting why XYZ doesn't work or why you're missing ABC feature and chances will be high it's because your template PHP file does not have the changes. For example next CB release is making some fixes to the template PHP file. If you supplied your own you would not have these fixes.