They do appear to help on windy days. Less water mist blowing over the driveway, so there is some savings in certain conditions.
The slower application rate helps with soaking and the ability to have larger zones due to lower GPM.
I switched from sprays because the zones were designed too large and there wasn't enough pressure to evenly cover the area. Rotary nozzles fixed it without splitting a zone and all the digging to add a valve.
But your right, they flow 1/2 the rate and you run them twice as long, so same water amount is going through the system. There is no magic, unless you can cut back because of better application.
In my particular case, I find rotary is applying it better, so I believe I am seeing some savings. I can water the same area with less gallons because the coverage is more even.
I do track my gallons/gaons per minute at the meter pretty regularly.
All cases are not equal of course.