Should workplace allow surfing of msn or facebook?

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I know many of my friends who make tons of complaints about their workplace disabling the msn or facebook function in their corporate internet network. In other words their office actually ban the usage of company's internet connection for other purposes except for work related requirements. Of course from the organisation's viewpoint it seems obvious why such sites should be banned during work hours as work efficiency may be vastly affected due to the freedom to log on to personal websites and leading to employees not able to work productively during office hours. Of course there is always the other school of thoughts which believes that in fact employees are more effective when given a certain window of freedom to surf these sites as a form of networking and a small break from their work which may ignite their creativity and productivity which may otherwise be stifled if purely confined to work related matters at all times. So what is your take on this? Should workplace allow surfing of networking sites eg msn or facebook? =)