I have RX 560 GPU & Linux Ubuntu 18.04.1 LTS;
I've used legacy linux drivers earlier (mesa or something like that i believe) and everything worked fine.
You have answered your own question
In other words: If you are not a Developer, Software Architect or Audiovisual Content Creator then you do not need the AMD Proprietary Drivers.
About your request:
when are you planing to provide us with some GUI software like windows users have?
IMHO If all the inhabitants of this planet with Internet access and having an AMD GPU can choose free and open software options is because thousands and thousands of Developers, including those of AMD, offer their efforts and knowledge so that we all enjoy free choice, consequently they are not obliged to offer us a GUI to configure our GPU.
All the best