Well, I've asked 2 separate times, so I'll ask again...
For the people who just show up w/ food, do you not worry that what you're bringing the host is already serving? I mean, what happens if you walk in the door w/ your baked macaroni & cheese & your hostess is just pulling her baked macaroni & cheese out of the oven? The hostess then has to serve both dishes while the other guests are forced to choose whose dish they eat.
I'm not uptight at all about stuff like this (at least I don't think I am!

), and, if you came to our house w/ a dish to share, I'd serve it because I wouldn't want to make you feel uncomfortable.
However, if I've already planned my menu & planned how we're serving the menu items (coordinated serving dishes & utensils, where the dishes are going on the buffet, etc.), having to stop & fit in your dish could cause me a little stress.
And, again, if it's the norm in your circle of family & friends to always bring a dish to share at a get-together, that's fine. But there are occasions where it's not appropriate.