IMHO, i think that there needs to be a seating arrangement. If someone comes late to a simcha, and there is no place next to people he knows, he shouldn’t have to feel uncomfortable sitting at another table, pulling teeth to make conversation!!! And some shy people might feel uncomfortable sitting by a certain table… Also, it makes a person feel that the hosts of the simcha thought about him.
But for the people who think that it’s easier to make no seating arrangements, go ahead. Remember, this is just my opinion!!!