The way I always thought of it is that the taskbar generally just can at best have a limited set of applications listed and takes up precious vertical monitor space, so its mostly limited to the overview/activities since that is the "I want to change apps" mode of the desktop and is just 1 click away (either super or top-left on the desktop).
Then again I am one of the (probably) few people that would probably even do away with the top bar currently still in GNOME and not have anything other than the app visible by default.
I use to be annoyed at the behavior as well when I started using GNOME, but at some point I actually started preferring it and now barely use the taskbar on Windows.
I remembered it as a version of what some Gnome designer claimed, but I tried searching for some statement on the topic and I couldn't find any explicitly mentioning it.
