I remember a discussion of why it was acceptable and even expected to be late at parties, but for work events, you had to be on time or better yet early, and the general consensus was "when you're doing work, you need to be precise, but when you're doing leisure, it's good to have time to relax while waiting for guests, and for guests not to stress getting there at a precise moment!"

I strive to be on time for work things, but don't mind being a little late to parties -- and this drives me and my my wife nuts, because my wife wants to be on time for parties.