I have two points to make:
Point 1:
I've known people rear ended while in traffic on the freeway. Even the best drivers get distracted at times and simply put the bike always loses. The car is a damn metal cage with years of safety devolopment. The bike might as well be as safe as a huffy. Having lane splitting will aid in bikers safety.
Point 2:
In California with the severe traffic 20/20 iirc (or one of those type shows) did a study on the time it took a Mail currier to travel a distance, one in a car thus stuck in traffic and two, a motorcycle. It took the car 4hrs iirc and the bike less than half that time. What does this mean for everyone else including car owner? Less traffic. For every bike lane splitting that is one less car making the traffic conditions worst for other car owners.
Tl;dr
Safety
Less traffic