At one point in development, I distinctly remember the riots were worth using. They dished out tremendous amounts of engine damage with great effect, immobilizing large numbers of targets, within a short time frame. However, I do not use them now for two primary reasons.
1. The effects on large groups of enemy ships seems negligible now. They simply don't have the stopping power they used to (maybe the change was made around the Unity switch, I don't recall). The riots take too long to do slow down large number or ships before they get to my important facilities (planets are not all that large, it does not take long).
2. The auto-build mechanic does not work properly. Specifically, it does not preserve different modules at the same tech level (for example, if I manually build one laser and one machine gun, if one is destroyed, it does not automatically rebuild the destroyed modules. I may end up with two lasers instead).
They seem a bit more effective if I move *really* close in range, so more of the guns hit, but if I'm that desperate to stop the enemy it is probably a wave of bombers or something, that are capable of destroying my riot when I move in.