That's why I ride with two. The lens angle on modern cameras is good enough to pick up cars approaching from the side. You don't necessarily have to show the vehicle approaching for evidence though - just the front facing camera will almost always show that you were obeying all the laws, were in the proper place and lane, were not running a red light, etc, up to the moment of impact, which will almost always put the truth to the motorist's usual bag of lies (He was all over the road, he veered in front of me, blah, blah blah).

I figure I'll be able to provide evidence for incidents that I'm near but not involved with too - I've witnessed several before I got the cameras and having a video record would have been helpful (especially in the hit-and-run that I witnessed, where I was more worried about helping the pedestrian than reading the plate number).