However, riots are spontaneous and so unpredictable so it will be some time getting the Guard on scene and organized prior to being effective.
The police can expect to be the only thing between the rioters and the public for a time.
Living here in Southern California in 1992, I know it was highly effective in the great L.A. hullabaloo. Although we're talking about two different scenarios and probably number of rioters and looters. But no one fussed about it back then I could remember. I don't think the unrest lasted more than three days or so. Of course, the military and police were using lethal force to quell the riot.