Applications should have title bars. This is how it's been done everywhere since pretty much the first Xerox GUI. Monitors are bigger than ever today. I promise, people can spare 10 pixels of vertical space so that they can see the name of the application being run (and the name of documents open in said application), and have a very easy area to click on to move the window around with. GTK3 CSDs also break the user's ability to choose a window manager theme. It also removes the possibility to disable the toolbar area from certain applications, eg text editors. They also tend to result in applications that lack traditional menubars, which are very useful for complex programs.
I have already dropped every last Gnome 3 application over this nonsense. This is going to be horrendous to have to stop using Xfce now, too.
Sorry, I know HN doesn't like negativity, but this "change for the sake of it" stuff is getting really old. I just want to keep using my computer the way that's worked just fine for me for the past 25 years. This really is a step backward in usability. I don't want to have to target tiny slivers between widgets, or hold down Alt to move windows around. And for applications I want an actual toolbar in, I don't like how most of the toolbar icons disappear so that things can be merged into the titlebar area.