Presumably the cruiser cameras are used to back up the testimony of the officer - at least that's what I've seen. They aren't used in a vacuum, like the street cams. Officer testifies "I did this and thus, and the perp did this." Produces cruiser video in support. Problem is that without other evidence, people can claim they were roughed up or evidence planted; with that camera, it's harder to do the claim.
Also, remember, the street intersection cams have been associated with deliberate shortening of the yellow light period. Of *course* when the yellow light interval is shortened, there will be more people caught in the intersection. Pure dollars at stake here.