For best security its better not to have much on the login or register page. Those pages should be separate from content and should use SSL if possible. In my opinion these are best practices.
So for example if it were possible for users to put content on those pages they could put images with src=http://someimage.png and now the user trying to login or register gets a browser warning about the page has some insecure items because the page enforces ssl but there is content that is not using ssl.
In my opinion its best to keep the login page and the register page focused on their purpose.
If you want to make a custom registration module or login module that can be put on pages you can do that, but it then it becomes difficult to ensure the data is submitted using SSL. One solution that comes to mind is maybe making a Silverlight login control, because inside the silverlight control you are independent of the main page form and can easily force an ssl submission even if the container page is not under ssl. I'm going to be experimenting with Silverlight myself and may implement this. So then if customers want their login page all decorated they can just put the silverlight login module on a content page.