For that matter, I still run one single Xt application: Emacs. That's because Gtk dumps core if its X session goes away, making the emacs server mostly useless: the bug for this has been open for years and appears unfixable. Now maybe this problem will vanish with Wayland (as it relates to Gtk keeping stuff on the X server but not keeping track of it so it can resend it if the X connection dies)... but maybe not. Demonstrably the Gtk hackers do not care about use cases like remotely running an application which maps multiple windows, then unmapping all the windows so you can suspend the machine on which the display is done.
Because, after all, saving power by suspending your machine when not in use while leaving your processes running on another machine so they can keep doing background jobs is a rare use case, unless you jam the word 'cloud' in there, whereupon it suddenly becomes immensely important. Or something.