HiDPI support on the Linux desktop is important for Retina MacBook Pros and other displays like the ASUS Zenbook UX301LA with a 2560 x 1440 WQHD display in 13.3-inches. Without HiDPI support, the desktop can be barely usable, as I explained with my Xfce adventures on the HiDPI Intel ultrabook in OS X Is No Longer On My Main System, But I Already Have Regrets.
While a bulk of the GNOME Shell HiDPI support already landed, it looks like there's still some last minute work being done ahead of the planned GNOME 3.12 release on 26 March. Pushed into GNOME Shell's Git today was a change to make sure GDK and Clutter scaling stays disabled when the scale factor changes and another commit pertaining to using the gtk-xft-dpi interface instead of monitors-changed interface for the scale factor. These patches are in regard to this GNOME bug with GNOME not handling the changes of the scale factor properly while the shell is active.