As far as the cheating thing, it's pretty obvious that banning them isn't going to deterr any but the most inept or casual cheaters, so it won't help much. However, why anybody would let a cell phone into a test is beyond me--some idiot always forgets to turn the thing off, and so you've got one guy's ringing phone distracting the other 29 people taking the test. Just like in theaters, movie theaters, college classes, meetings, and all the other places where you're not supposed to be making noise--it's rude on top of annoying, and they have no business being there.
Studies at places like Harvard have linked somewhere around 3000 annual deaths in car crashes to people distracted because they're talking on a cell phone. And why? Because they can't wait the fifteen minutes to get to the mall to continue the conversation? Yes, some people have two-hour commutes, but they're not usually moving for most of that time, anyway.
And yes, it's legal to drive with a sandwich in your hand, or to talk to a friend who's actually in the car, but there are two significant differences: The sandwich is taking one hand off the wheel, and making you more dangerous, but it's not specifically distracting you (unless it's REALLY good)--you just happen to be chewing while driving. And, the guy in the car with you is talking to you, but at least he's in the car and as such aware of the conditions, so he can adjust his conversation to the driving conditions, as well as point out potential hazzards if you're not paying as much attention as you should be (I know I and the other drivers I know do that--it's safer to have my wife riding shotgun, not more dangerous, even when I'm talking to her).
Anyway, we're evolving into a society that can't make a distinction between the appropriate use of a cell phone and actions that're uncourteous or dangerous.