I thought this would be an 'ask hn' type thing, but it appears to just go to a marketing page.
I'm not sure there's much of a compelling reason for using Silverlight or Flash or JavaFX for front-ends in most cases. If you have a particular design or interactive needs and there's already a library or component that works in the particuar front-end tech you're looking at (a silverlight charting lib, for example), then it'd probably make sense to use silverlight. However, that's mostly from a pragmatic cost-only view, nothing to do with the technology qualities itself.
From what I recall, Silverlight does make it somewhat easier (compare to Flash) to interact with javascript on a page where the SL app is hosted. However, javascript interacts with javascript even better.
The bigger question to ask, imo, is are there things you can't do in JS that you need to? If so, use SL/Flex/whatever. Realize that you're likely limiting the audience somewhat, especially if you're going the Silverlight route. However, doing cutting edge JS is also going to limit the audience somewhat too, as not everyone is running (or can run) the latest browser versions.
FWIW, 3birds, if you're reading this, come on our to our PHP/MySQL meet up next week - we're in Carrboro, and you're in Chapel Hill :)