Hmmm. As someone above said, surely that depends on whether you use your Mac for things other than Cubase? There are a number of very useful updates to El Capitan (crap name - agreed!) and some things (wifi drop, iCloud sync) that never really worked properly for some under Yosemite.
Additionally, if you are immersed in the Apple world you need El Capitan to ensure you get maximum compatibility between OSX and iOS 9.1.
I suspect something is being lost in the German to English translation but I find Steinberg’s response lacking.