There is a difference between "Oh I can't talk about it we're in stealth mode" and not running an ad campaign notifying the world of what you plan to do. As a start up Google and MS literally pay no attention to what you are doing. It is only after Apple picked them up that the big G even bothered attempting to put together voice search on android and did it relatively quickly.
what benefits did they derive from stealth? None as far as I can tell. Google didn't care about trying to re implement their idea until Apple showed interest and they needed a counter to Apple. As a general rule of thumb no established company is going to steal your startup idea until after you have launched and proved successful, pre-launch they have better things to do.
What did they lose by not talking about it? Who knows people around here like to talk about the serendipity of bumping in to the right person while at lunch.