One could say that it's a cheap way to test a product.
WLM is a product that will be used by tens of millions of people.
Finding bugs in a small group (i.e. Messenger developer team in Microsoft) would be an enermous task.
So, they decided to release a beta version to see people's reactions about the beta, get feedback and bugs submitted by WLM beta users.
In fact, some people are obsessive with beta (read: buggy) products.
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