The Autobahn is much safer than US highways and there isn't a speed limit. Speed in itself isn't dangerous. Going significantly faster or slower than the prevailing flow of traffic is what is dangerous. As far as saving time goes, during urban commutes, the time savings is negligible. On a 500 mile road trip going 75 vs 55 mph will save you almost 2.5 hours.
Speed limits are about revenue. Plain and simple.