Moreover, the Watch may even help Apple to rival Google when it comes to Siri and the cloud: the best way to improve a service like Siri is to have millions of customers using it constantly, and I for one have used Siri more in the last two weeks than I have the last two years. Multiply that by millions of Watch users and you have the ingredients for a rapidly improving service. Perhaps more importantly, the fact that Siri is critical to the Watch’s success in a way it isn’t to the iPhone’s may finally properly align Apple’s incentives around improving its cloud services.
Apple has been improving Siri both in terms of speech recognition and load times considerably over the past two years (they're now at the third generation of Siri). I'm finding the wrist to be a better activation point for Siri – raising your wrist to talk to a watch like a spy somewhat feels more natural than staring at a phone and speaking into it (although that may come down to cultural heritage and personal taste).
As I wrote in my iOS 9 wishes, faster interactions with all Watch apps (Apple and third-party ones) could be possible with a Siri API. I'm curious to see how Apple Watch will shape Siri's future.