spoiler space
So, are they pretending that Ben doesn't exist anymore? If the kid from Big Daddy wasn't available, there's no reason not to recast. It just seemed very strange that he wouldn't be there. Same with Rachel's Mom and Dad. If Marlo Thomas and Ron Liebman were unable to guest star, a reason for their characters' absence (mom's got the flu, dad's out of the country?) would have been nice.
Was the party a last minute thing? It was rude of Rachel and Ross to expect everyone to be there after they had already made plans (especially Mon & Chandler's vacation). I'm probably taking this show way too seriously, but the fact that R&R would do that pissed me off.