Do you drive in town? I've lost count of the number of times I've thought: "If only I had more coverage to catch the complete stupidity of what that person did."And if said person hits me, you can bet I'd want proof of it all. But more cameras is the way to go.
I only ever drive urban / sub urban. A front camera is for the front and 3/4. For 3/4 you only need a few feet either side. You don't need what the driver did prior to leaving his lane, only you were in yours and his car (with number plate or face in view) left his and hit you. If you want side coverage you want side cameras as you said, but it all becomes excessive in my view. Even a front camera, for a side impact, if it shows you in your lane, captures a bang and you swivel the camera to show his car hit yours, you have evidence. You have to ask, what are you trying to prove, crashes or get footage of poor driving to report other motorists? Personally I've little time for the latter do gooders - it's not what dashcams were designed or intended for. Policings for the police. Dashcams are for crashes.
Even with a corrected linear lens, the wider you go, the smaller the objects appear (because they're in a bigger landscape), and so the harder it becomes to retrieve detail.
This video shows you get a perfectly good view with only 70 degrees, although I would go a little wider:
2 still frames captured from the video, 70 degrees top, 120 degrees bottom (captured from different time points as the guy doesn't appear to have aligned his video properly - I've tried to go off the road markings straight in front whilst allowing for 1 cam being mounted higher than the other).
Looking at the car to the right, if he were to leave his lane, 120 degrees offers little benefit prior to a collision with that vehicle (you not really capture anything else - an extra 30cm (1 foot) of car)), and there's a clearer less distorted picture from 70 degrees. The view in front is also clearer on the 70 degree despite not suffering distortion from being at the edge of the lens - the benefit of objects being captured larger in a smaller picture.