It is a good practice to have the entire content on a website be protected through SSL. In the case you have mentioned the following have occurred:
- The SSL connection handshake have gone successfully with the browser and server. The negotiations and certificate validations are performed as required.
- It is ensured to the user that he is browsing the page hosted by the party it is meant to be.
- Not all contents which are visible in the page are from the URL. There are some links in the HTML code which points to some other servers (or same server linking to static reference in different context) in internet from where data like images or text are fetched and displayed.
- The important thing is that the user name and password you are going to type shall be sent to the server in SSL session only and in enciphered format which is very important.
As I said earlier, it is not the best practice, to increase the faith and confidence of the user, it is recommended to have all data displayed in the page to be loaded from the same server using the SSL session.
The draw back is SSL puts overhead to the data which is not desired for static images and text which are not of much importance. It depends on the business requirement. If millions of people are hitting the page everyday than it shall matter to the business in terms of the bandwidth used by the servers. Typically Banking sites will never follow the approach of partial data in SSL and some in non SSL as it can severely impact the user response.