I wouldn't make it a rule either, but I do agree with it. About half of my competitive costumes were not worn in the halls until after the Masquerade. A lot of my competition costumes were uncomfortable enough for the event that wearing 'em all day would have been ridiculous. And some relied on a bit of surprise with the skit.
A few exceptions would be my most recent costumes with my martial arts buddies, since we had fun as a group and wanted other folks to be interested in seeking our skit. And I can think of a couple more of my less complicated cosumes which I wore in the halls prior to the event, but that was due more to the inconvenience of changing mid-afternoon than for wanting to keep the costume a surprise. I think it really depends on the effect you want for the audience.
As for seeing that unspoken rule in action, most of the costumes I saw on stage with my bf at CostumeCon18 weren't worn in the halls either. Quite a few of the entries relied on the stage effects to get the full impact of certain costume parts on the audience. Some costumes were amazing, but would not have the same impact in the halls just for what they were. For example, the woman who won Best in Show that year made herself into a shiny, glittery lily pod that opened up with her built in as the Lady of the Lake, complete with Excalibur attached to her front. It was a gorgeous costume and she did a very simple presentation with lighting, recorded dialogue, a fog machine and simple movement on stage. The stage presentation made the beautiful handiwork into something really inspiring. But it was a hot, stuffy costume that would just get a lot of questions just standing in the hall. Like, "What are you...? Ooohhhh, I get it..." Which I can imagine will get annoying after the 20th time.