When it comes to legal protection, one front cam does 98%+ of what needs to be done. A rear cam can prove a wild driver's behavior was occurring before they caught up with you from behind. Side cams may show them gesturing, swerving etc, but also may not show your lane position very well again leaving the brunt of the work on the front (and rear if you have that) cam. They can also show approaching cars at "tee" intersections and 4-way stops which you may not get on a front cam's edges unless it's got a wise enough lens angle.
My cams also provide a security function for me, but my situation is far from the norm. I work in places where unwatched work trucks get items stolen from them, and having visible side and rear cams helps with that. As long as they think they're being watched they leave me alone
Phil