A common interchangeable language for both client and server side.If you data-intensive app produces complex visualizations, this can provide a big boost
There are a lot of things that you will find very valuable for a data-intensive application when using SL (many of which I believe are impossible in more traditional web languages): (you can look at my other questions to see the silverlight cpu usage question) I have had some rendering/performance issues on the mac though - nothing that made me regret using SL.
From a data-intensive app standpoint, there is really no difference between the mac and the PC. One began as a SL2-B2 app and shipped in SL2 and the other is a S元 app. I have developed two data intensive applications in Silverlight.
