When they are in public locations, hiding username, and avatar will be good for anonymity and some of forum members will like this extra feature from SMF patches. It's extra feature for personal choice so if you don't like it, just use default features and settings.
Username won't be hidden when you shrink the header, only avatar profile image will be hidden with total time logged in, and few more lines of text.
Everyone can still see that your account is logged in, but you can always log out if you want to hide that information and only read posts.