Since I'm also using the legacy over the default reader, the reason I do it is because it loads ALL the pages, not just 20 like the new one does. I want all the pages to load before I start reading it.
If you allow us to load all the pages on the new one too, I'll start using that instead.
Also the arguement "don't using because it's not supported" is a bad one. Just like Windows 7 and Windows 10. 7 is better but people use 10 because it's the new one.