Virtual Worlds are fascinating in the sense that they are the most realistic model of the real world in existence today. Due to that closeness they can be used in many ways. Similarly to social networking sites, virtual worlds allow users to interact with each other, although these interactions seem much more real since there is a stronger visual aspect when seeing someone else's character. This can have the same drawbacks as social networking sites in the sense that it is still not the same as real interactions and so, in my opinion, does not contribute to strong relationships like those that development during real life interactions.
Virtual Worlds also allow people to act in ways they might not be able to in real life. Any mental or physical limitations that people have in real life can be forgotten in the virtual world. This could work as a pro or a con. These people are able to experience things they cannot do in real life, which can either help them feel relieved that they are finally doing the thing or possibly even more frustrated and angry that they can't do it.
A final possible way for virtual worlds to be used that I will discuss is in the business arena. Mark Tuttin writes in his article "Going to the Virtual Office in Second Life" about how companies are now using virtual worlds to hold meeting and literally do work in a virtual office. While this could be a pro because their is much more flexibility as to where employees have to live and the amount of office space needed by a company, I think it is ultimately a con. In my opinion people work most efficiently in an office environment where everyone around them is working. In a virtual office people can be sitting at home distracted by a tv or by their kids. Also the relationships that develop between office workers would most likely not be as strong.
I think with the advancements in 3D technology the future of virtual worlds will be even more realistic than they are today. However, I think the business spectrum will shift to video conferences as opposed to virtual conferences, also due to 3D technology. Imagine an office meeting where everyone is there via video feed and everyone looks 3D...amazing.