Surprising to some, but GUE's "principles" do leave some things open to flexibility. Some divers prefer to keep the spool and SMB pre-attached. Other divers prefer to leave them separated for various reasons. One reason I think I recall an instructor mentioning was flexibility; maybe when doing a certain kind of dive a diver might anticipate a possible use for the spool other than attached to the SMB.
I pre-rig my orange SMB to my reel so I’m not eating away at my planned BT. I leave my yellow SMB separate from my back up reel so I can send it up the same line (primary reel) if necessary to deploy a second DSMB.
I see the value of leaving both separate during training so that the trainee learns appropriate problem-solving.