CSS has natural fallback mechanisms such that if the browser doesn't understand a property:value combination, then it will ignore it and use something declared before it if there is anything, thanks to the cascade. Sometimes that can be used to deal with fallbacks and the end result is a bit less verbose. I'm certainly not a it's-gotta-be-the-same-in-every-browser kinda guy, but I'm also not a write-elaborate-fallbacks-to-get-close kinda guy either. I generally prefer a situation where a natural failure of a property:value doesn't do anything drastic to destroy functionality.
That said, @supports certainly has use cases! And as I found out while putting this post together, plenty of people use it for plenty of interesting situations.