So does your (lack of) knowledge of the products it seems.
If you call someone and get diverted to voice mail, you still pay for the call at whatever rate you're on.
SVS is (will be) charged at a fixed rate and is available to all VC customers, not just those who have voice mail boxes configured (quite a small percentage). It's also easier to use than voice mail.
The whole idea of SVS is to give SMS-like capabilities to those who cannot easily use SMS (think about this before shooting from the hip) or those who want to convey more in a message than possible just with written words.