Only when you want to run it on real hardware. Most qemu targets can boot straight from a binary blob or an ELF image (the emulator initializes the emulated hardware and then sets the PC to the address where the blob is loaded). Those that can't are easily modifiable to do so.

Also, U-Boot can boot a lot of things this days.

So if I were to build an OS for ARM or MIPS I would:

1) Build it on an emulator that can boot from ELF or a binary blob.

2) When it's time to run on real hardware, try to use U-Boot.

3) If that fails, write my own boot loader.

The moral of the story is: bootloaders are not my kind of fun, so I'll avoid writing them at all costs.