Should employees be blocked or allowed access to LinkedIn

I am wondering what the consensus is in regards to blocking access or allowing access to LinkedIn for employees on their work computers? Personally I do not see a real business reason for it though I do view LinkedIn as a good means to connect with other professionals in your field. I feel that this opens the door for employees to shop for other employment opportunities while on company time. While I do advocate this website as a great means to network but I just cannot see how this could be work related in any way.