If you mostly use GTK applications, it's a great way to use them in a well-rounded desktop environment that isn't Gnome. That's appealing enough for a lot of people, even 10 years after the first Gnome 3 release. Some people (like yours truly) just ditched the applications and moved to other things, like KDE land. Some didn't and just ditched the DE.

They have very legitimate reasons -- e.g. Thunderbird (and Evolution, for all its faults) kindda work with Exchange if you jump through some hoops, whereas KMail 2 still barely works with anything.

Other than that: it's very stable, very fast (QML and QtQuick, as it turns out, aren't very good at giving you a fast, low-latency interface and permanent iteration by lots of developers), easy to setup.

In many ways, XFCE is to Gnome 3 what KDE 3.x was to KDE 4.x and the Plasma series (except it's also maintained [1]): a solid, traditional platform that modestly aims to build things with the tools we have right now for the computers we have right now, instead of developing an advanced technological and design framework that will also work on the machines that the design hive mind says we'll have tomorrow.

[1] Yes, I know about TDE :-).