Since the early days of the iPhone, we've received a steady stream of complaints about how alerts are handled when you're on a call. Namely, you can have the phone to your ear, chatting away, when suddenly a text message comes in and the tone and/or vibrate goes off full blast into your ear and rattles through your skull. Since push notifications launched for 3rd party apps, especially the trumpeting horror that is Game Center, it's only gotten worse. If you're not used it to and expecting it, it can make you want to drop your phone and start stomping. That's only mild hyperbole, mind you, as that's exactly what several significant others have told us, with rage-filled eyes, after it happened to them for the first time.
While some people may want to make absolutely sure they don't miss an important alert while on a phone call, and are willing to pay the price in low-yield skull trauma, others would pay a significant amount of money for the option to turn it off.