If we said that people coming to Brickworld had to put all their MOC's into certain places with other like creations, that would probably deter some people, since we've been doing it the other way for 4 years.
Honestly? If that is all it takes to keep people away, then I'm sorry but it seems rather petty. I've been to Brickworld and Brickcon. I had both my own table and worked in one of the theme areas at Brickworld. Mostly because the MOCs I brought for my table, I didn't know what theme they would belong in. But I do prefer having it displayed in the themes much better. People get time to talk to each other, work together to created a cohesive display,which is SO much fun, and you get to meet new people who you might not have spoke with before. Which to me is why you go to cons in the first place, to meet friends, make friends, and build amazing things together.
Also, lets just take me as an example. I do not build in the Star Wars theme (long story not worth mentioning here). I'm not interested in the Star Wars theme. And if I don't want to look at the Star Wars theme, well if there is a set display for it, I can avoid it very easily and spend my time looking at the themes I do like. Which, while that is a bit close minded, is a bit helpful. Now train, that can be sticky, because they come with their whole set up ready in advance, but if they have their set ups in one general train area, then this could also work. In other words, have a whole room just for train since Brickworld has a lot of train displays.
Just because something has happened for four years, does not mean it has to continue to happen. Make a formal survey. Collect the data. Because you might just notice that you are talking about a handful of people.
I guess it goes like this, if Brickworld wants it like it is, then fine. But if a few people have to let their egos keep them away from a con because their MOC's have to be put into a theme area, well then what is the loss of a few people when there are over 750 attending that event?