This is not from Microsoft. This is not official statement. But, I think this is what MS ment.

Silverlight focus shift = less use of Silverlight on MS websites, aka Bing Maps. The main thing is they need to push new features that are accessable to non-SL browser users. And Bing Maps particular has to make sure that is true. Because use two versions of Bing Maps reallyconfused users.

Although I have to say, this is too early to make this statement when people don't have HTML5 browsers. Silverlight is much consistant if you want to do a feature rich map. If they really want to cater majority of browser users without plug-ins, should use HTML4 instead. More like, hey, we are pushing HTML4 LOLz.