X forwarding seems to work enough for my primary use case. I have some supermicro servers for work with a java applet for KVM. Because of a series of reasonable but unfortunate policies, I can't run the applet directly in the laptop's host OS. The best way is to run it in a VM. It's a lot better experience for me to run the applet over X forwarding despite some issues with window resizing that I suspect are related to hidpi, than to run a full X desktop in the VM. Host is osx, guest is freebsd with linux compat and oracle jre for linux.
It was also useful when I wanted to output some pixel graphics from Erlang to my Windows desktop from a program running on unixy devices. x server windows, x client on linux, freebsd, os x -- occasionally also ran with os x as x server and linux or freebsd as x client.
The key thing for me is the ability to export the gui for a single application on one system to a desktop environment on another --- I think this is possible with other systems, but it's not easily exposed. Performance and fidelity of the experience is usually not that important for me, as long as it's usable.