Xubuntu doesn't. 4.12 in 15.04/15.10 was much the same as 4.10 in 14.04 (other than bug fixes and some small tweaks to fix stuff they where near identical).

The XFCE folks reached rough parity with Gnome 2 in terms of desktop usability then just focussed on small increment improvements and bug fixes and that has been a major win imo as I can use a desktop that supports modern linux distro's, is fast on everything and rarely surprises me.