Windows 8 has a special version of Windows Defender, which is far more secure and stronger than prior versions and certainly better than Microsoft Security Essentials as well as the original Windows Defender used in Vista. That being said, it is not perfect and there are viruses it cannot defend against or if it can defend against them, it has trouble defeating them completely. It is certainly unable to completely remove a Rootkit and thus far, Kaspersky's TDSSKiller is the only option I have found in that regard that completely removes a rootkit forever.
In regards to your question, "Does Surface Pro need anti-virus protection beyond what Windows 8 provides?" This is really something that is up to you and depends on what type of Internet browsing you do. If you are continually going to dangerous sites and visiting sites that are known to be corrupted with dangerous items, then yes, you definitely need something beyond Windows Defender. If you do light browsing to well-known and secure sites, don't click on the pop-ups that say, "Click here for the best male/female enhancement!" and in general, do not visit places that Google or Bing shows in a search result as dangerous, then you will be okay.
Again, this is completely up to you but the built in anti-virus is very versatile and definitely the best anti-virus Windows has ever built. That combined with SmartScreen and the other technologies used by Internet Explorer, you are much more protected than in any other version of Windows.
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