For me to consider a technology that requires a client-side platform, I'd probably would like to see 50% or more of adoption on our user base. Silverlight at this moment has close to 0.00001% (give or take a few digits).
Microsoft did other stuff on that past that didn't get any adoption, so, if they fail with this one it won't be the first time.
As a startup, every bet you make has to bring significant return and move you forward. A single bet that doesn't pan out is enough to kill your business. For now we are sticking to well-known, well-established client technologies like a browser, HTML and Javascript. Turns out that 100% of our users have that and near 100% of the Internet population have that as well.