I have it turned off as well. So far as I know if nothing else it can be used to transcribe your words in a voice-to-text sort of way. We here on slashdot may be good touch typists, but some people can't and/or won't commit to typing. So being able to talk into the machine is helpful.
And I guess MS's assumption is some contingent of users have been only using iphone for however long that group may be accustomed to only using voice-to-text. If that were the case said people may find trying to use an icky QWERTY keyboard alienating. I imagine if I were say...14 years old and had used very little iOS 99% of time my whole life being forced to switch to typing on a windows computer could be quite alienating. Like something out of era of bell bottoms (thinking like this imaginary 14 year old for a moment, I lack imagination).
Of course, I can think of other reasons. China and india are big markets for instance and most of those populations have only known the phone as their sole computing device. A voice interface of some type might make this transition much easier.
Lets face it: the markets of "the west" are second fiddle now-days. It's more about accommodating the up-and-coming potential markets. If that were the rational I don't fault MS at all for it. I mean 90%+ market share in the western hemisphere there's not much place else to go but China/India that's like 2+ billion people. Choice of who to cater to seems obvious.
Obviously I'm just speculating here. And I hope I had answer to your question somewhere in there....