I'm inclined to agree that an after-boot firmware interface is unnecessary; I don't see the point in keeping OpenFirmware alive after the system is booted -- but OpenFirmware does seem to be a good choice as the boot firmware. It's a lot smaller and easier to deal with than Linux-as-bootloader, which we were using before. We only have 1MiB of NOR flash to boot from, and L-A-B was too big. OpenFirmware does the job very nicely.
It's also been a godsend when we've been bringing up the hardware. We've designed entirely new chips for OLPC, and have been working with FPGA versions of them during their lifecycle (the final build with the ASICs of everything should happen this week). Debugging that hardware with OpenFirmware is like magic -- especially if you watch Mitch doing it :)