Here's an abstract from some studies published by U.S. National Library of Medicine http://www.ncbi.nlm.nih.gov/pubmed/20927736
MAIN RESULTS:
Thirty five studies met the inclusion criteria. Compared with controls, the relative reduction in average speed ranged from 1% to 15% and the reduction in proportion of vehicles speeding ranged from 14% to 65%. In the vicinity of camera sites, the pre/post reductions ranged from 8% to 49% for all crashes and 11% to 44% for fatal and serious injury crashes. Compared with controls, the relative improvement in pre/post injury crash proportions ranged from 8% to 50%.
I'm not very good at interpreting study results, but in this case it's obvious that speed cameras help save lives. The source seems to be trusted also.
For what it's worth, the German government is one of the best out there, but as any other government it doesn't care much about well-being of its citizens. Maybe, that's why they don't put cameras everywhere. And because people generally don't like speed cameras, nobody pushes for their deployment all over the roads.