And we're going to assume that sending out up to thousands of texts per month, worldwide, would be free? I somehow doubt it.
And I suspect that if people badly wanted those vibrations, they could sign up for twitter and use some app to automatically tell them of updates.
Now, you're bound to start complaining "not everyone has twitter" again, but if you want extra features, then you'll have to put in some effort.
Also, texting suffers from the same problem email does: it requires you to sign up (your phone number, even).