I know at least some people will build a list when they get there. Not because they can scout out what people are playing but because they just like building lists on the fly. Or perhaps some thought occurred to them on the way to the store and they want to try it out.
Most of the bigger events I've been to have been so crowded, it would be terribly impractical -- and borderline rude -- for everyone to bring everything they owned and build their squads there at the store. I always just bring what I plan on flying that day, with some replacement parts just in case (mostly extra pegs and bases). I can fit everything in a backpack and not need half a table for two tackle boxes and my card binders.
Edited by DailyRich