To get right into it, right now piracy is still the main issue. Without a 4S jailbreak there is no way to obtain the necessary files without an actual iPhone 4S and a developer certificate. Distributing these copyrighted files would be illegal.
The second reason is that it really isn’t practical for every day use. The validation data required to authenticate with Apple’s server expires every 24 hours. So every 24 hours you need to have your friend help you get the refreshed data. Also, it’s not like your getting your “own” Siri. Your device and the device you go the data from cannot use Siri at the same time. If you do, you’ll both just get the spinning purple light around the microphone. And in my testing, Siri on my device randomly reverted to calling me by the name of the person whose data I’m using.
While a Siri port will not come from me or the other people who have gotten it working this past week, I am certain that we will see it from @chpwn and @stroughtonsmith most likely once the 4S jailbreak becomes public.