Why should it matter? Silverlight is a free plugin supplied by Microsoft. Surely the intended parties can download it if they don't happen to have sl installed. It's an automated process and hard not to know what to do. Also don't forget that this is fallback, so either the end user upgrades their browser (in most cases involving ie6/7 this is not possible) or they install the plugin which is free. Your question is pretty moot at this point because sl has a 60% penetration rate which is not the same 90% of flash but still significant and quite sufficient to invalidate your argument. What if they required silverlight somewhere else? what if this is a corporate environment which is usually 90% of the case and normally this is the user group having trouble upgrading from ie6/7. In these environments it's more common to use sl which is used a lot for LOB apps and the probability rate of them already having/wanting sl is much greater.
Edit: forgot to mention WP7. This will work on there while flash won't.