If you're aware that Siri is "mostly a thin client to Apple's services", why would you expect it to update with iOS releases? If it's a thin client, it can be updated (or not updated) at anytime on the server side.
[1] On new functionality that requires resources installed on the phone (the new sports scorecard things, for example), I understand that only happening on new releases. When I say "take me home" and it only starts navigation sometimes, it clearly has the ability to start nav based on something, so I expect that to happen more reliably.
If you are in Canada and ask Siri for something like "where's the nearest coffee shop", in iOS 5 you get "I don't support that in Canada", but in iOS 6 you get the expected results.
I'm sure the back-end processing is the same, but because 6 supports or will support new commands, then they are running separate instances & databases which results in varying update schedules.
As long as they don't restrict based on device model (only iOS version) then I'm happy with that.