Google has managed to add the entire Google Voice Search, which is not dissimilar to Siri, to Android and the entire thing is usable while offline. This is how it should be.
I don't know where you got your information from, but this is my Nexus 4 without an internet connection using voice search:

Voice recognition is not an easy task, you need vast amounts of data and a lot of processing power if you want to it to work with a majority of people's voices.