Interesting issue for sure. I understand from your point that it shouldn't matter. I don't really think it does. If a spots taken then it's taken. It shouldn't really matter if it's popup or a 40' fiver. However, from my perspective when I hit the campgrounds and there are only a few spots of many that I can back my 34" trailer into and I see one taken by a popup then it does frustrate me sometimes.
I try to plan by reserving spots that I know I can get my trailer into first. It's simply my fault if I show up and there are no spots available for me. It is still frustrating though to see 10 spots open but too small for my setup and then 2 spots that would work for me but instead being occupied by really small trailers that could have fit into any of the other spots open.
In the end though a premium spot is that. If the roles were reversed and the best spot (say right on the creek) was open and I made a reservation for it then regardless of what size rig I had I would still be in that spot.
Thankfully I don't have these issues most of the time because I mostly boondock except for a few times in the State Parks each year.