According to this post, Jeffrey Tippet (Microsoft employee) writes:
WEP can run in two modes: shared and open. Despite the names, shared is actually less secure than open. Because of this, our telemetry shows that that only 0.1% (and declining) of Windows 7 users connect to these highly-insecure shared WEP networks.
In order to simplify Windows, make it easier for people to pick more secure defaults, and to enable us to focus on improving the wireless networking that the other 99.9% of our customers use, we removed shared WEP from Windows 8. Windows 8 continues to support open WEP. We recommend that you configure your AP to use open WEP, as it's more secure and more broadly-supported.
However, there isn't an explanation as to why shared is less secure than open. Why is this the case?