Thru hikers are people committed to completing this one trail within a year, if we follow the standard definition, and I guess they evolve to 'thruhikerdom' if they meet this definition. Unless sleeping in a certain number of shelters is required to meet the requirements, it's unclear why they should receive preference at shelters, or anywhere else. Perhaps an exception can be made in areas with limited tent sites, but otherwise they should simply use their skills to complete the trail. (Also group camping at shelters seems to not be a good idea, if I were a group leader I'd avoid this because it would exclude the random hikers passing through, out of courtesy). Otherwise we need to set up something like a European royalty/privilege system that anoints the intended thru hiker with special privileges and requires other hikers to defer to them. We should all just get along and enjoy hiking in the way we define it, not defined as by someone else.
YES - with the exception of groups - and by groups, I mean scout/youth/family groups of four or more. Two years ago, I was in GA, and there was a family of 7 hiking - staying in shelters. I was so happy that I could put in more miles to get the heck away from them!
BTW: I always bring a tent or hammock to use if shelter is full... but I prefer to be in the shelter as it is easy-peasy for set-up and take-down....
If one is a hiker then they are never through hiking.