GRUB offers automatic EFI installation with configuration generation and so on. Installing GRUB on UEFI is not more complicated than installing it on a BIOS.... normally! GRUB needs a working UEFI System for this, so GRUB needs a working Live System. If you dont have this, you have to use painfull, error prone workarounds to archive Gentoo installation on UEFI. As GRUB offers easy and automatic support for UEFI installation and nearly all new systems got UEFI Support, the Live Image really have to support UEFI. Some newer systems even dont have an option to disable UEFI (or you run into heavy bugs if you do so), so its aboslutely nearly impossible to install Gentoo on those systems with the gentoo live image. A good example is the Archlinux Live Image with its UEFI supports. Its easy and uncomplicated to boot this Live Image on UEFI systems and installation of GRUB and booting works like a charm. I think its essential to have this feature on Gentoo, no matter if someone likes or dislikes UEFI.
*** This bug has been marked as a duplicate of bug 370331 ***
