Text messaging has enormous utility in a lot of situations. I always text my flight number and airline to whoever's picking me up from the airport, for example. Otherwise, the person would have to (a) hear me correctly over the cell network and (b) write down what I said on a piece of paper. Texting is just better in general for sending any kind of data where it's important to be precise (flight numbers, addresses, phone numbers, etc.). It's also a lot easier to make changes to plans through a mass-texting. Instead of calling all your friends and saying, "Hey, let's meet at 8:30 instead of 8:00", just send them all a single text.

Honestly, SMS and MMS are there as a stand-in until getting push e-mail on phones is more common. Right now, e-mail is pretty much the domain of the smart phone. If and when dumb phones get the capacity to do e-mail relatively well (not inconceivable with Android), text messaging usage and rates will probably diminish.

The perception of Apple (which is what the stock value reflects) is dependent on the health of one man. While this is a concern, it is not the same as the company itself being dependent.