It's not the recommended setting as most people have additional fields that they want completed and with that setting turned on, someone could go to wp-login.php and register with just an email address.
As far as WP and WP-Members are concerned, a login is a login, so it doesn't matter unless you are trying to make sure you have certain registration data (which is sounds like you are not).
Incidentally, the extra fields that install with the plugin are just an example/starting point. You don't have to use any or all of them and you can create your own.
BTW... the "free" version is the only version. There is no paid version of this plugin. There is a paid premium support site that includes support, tutorials, and some premium add-on modules. But the actual plugin is only available as a free version.