Obviously it's a heavy weight solution, but the idea of having more drivers and better support is compelling— it's a much easier path to exotic boot setups, eg over wifi, off a SAN, whatever.
So...this is strictly my own personal experience, but, I've always had better compatibility with my hardware in the BIOS than I have had in any OS. I can't count the number of times my K/V/M has worked in the BIOS, then suddenly stopped working once DOS/Windows/Linux started booting.
But EFI can be implemented by others. Including U-boot! Which is really lightweight.
UEFI DXE loads disk/network -> Linux
We're doing Linux (with commonly used open source drivers)-> Linux
It's possible that if your end goal is a Linux system without too many requirements, you could stop at the first Linux system without ever booting the second. However the bios rom memory is pretty small, and we're mostly working with about 4-8mb of free space left not including the ME region or UEFI PEI.
Someone did that! Well, something like it, anyway.I have an Asus EeeBox sitting on my desk, gathering dust, that has something called "SplashTop" - if I hit the right key during the POST screen, it boots into a minimalistic Linux desktop in about 2 seconds.
There is networking and a browser. I haven't used it in ages, but I think the problem was that it could not access the file system, so one could not do very much, and I think settings did not persist across reboots. But the general idea seemed appealing at the time.