I totally agree: let’s use Blender tools if it has is! It will always be better: button, dropdown, collection lists, etc.
But Qt has some advantages too, because you can create tools with any design you could imagine (image viewer, custom editors, …)
I’m sure that other python GUI framework should work in Blender too like GTK but I never tried it and Qt is already used in the industry like bsavery said. Qt tools could be shared with Nuke and Maya I’m also working with and having it from Blender is an advantage.