Booting up is a bit of a special case that I'd actually be willing to carve out an exception for. When the boot-up goes wrong, it's often due to causes so deep that they leave you with very few means of diagnosing it or showing an error message. If the booting process hangs, having a rough idea at what stage it happened can be very helpful. Without these messages, discovering the cause of failure would be much harder.

The boot-up splash screen hardly even counts as "GUI" anyway.