It's rude if it's intentional, but sometimes the problem is, you just don't know who's coming. That's why I hate buying food for post-recital receptions, for one thing--they're open events, which means the whole world could show up, or just your profs and your two or three best friends (slight exaggeration there, but still). I imagine other events are the same.....a "family" barbecue, for example, could translate into Cousin Murgatroid bringing his new girlfriend, or Uncle Mack bringing his whole bowling team, so all of a sudden, if you'd planned on, say, twenty people coming, and you purchased forty hot dogs.......you can see how a larger-than-expected turnout can lead to problems.