The laws have not caught up to the research showing that even hands free calls cause problems. It turns out that most cell phone calls cause concentration problems. Technology has improved and we can multitask more, but our brains haven't changed.
Again, conversations or arguments in the car do not cause the same problem either because the passengers are paying some attention to the driver's situation and/or because we visual people we are talking to but cannot see.
http://www.nytimes.com/2009/07/19/technolo...acted.html?_r=1
Just because there are other bad things that people do while they drive does not mean that we should ignore the problem of drivers distracted because of cell phone use.