I think it is rude.
Not when someone is held up by circumstances or misjudges the time but when it's just because people don't give a **** it's rude.
I belong to a community (kinda) where it is a cultural norm to arrive at functions late. I've also seen the chaos and complications this causes for the people trying to organise functions especially people who don't belong to the community and assume that if a function starts at x time then they can bring entrees out 30 minutes later only to serve them to a half empty room.