[AragornII]

First, how do we know that everyone was there before Frodo and Co. came? By the movie, nobody came until after Frodo was healed. So, by the movie, even if he hadn't been stabbed, they still would have had to wait. Second, in the book, after the council they sent out scouts to find out was going on outside. They stayed in Rivendell for two months before they set out, so that three days or so wait while Frodo was healing wouldn't have made much of a difference. Third, even if they had set out earlier, they still would have hit bad weather on Caradhras because, if you go by the movie, Saruman was controlling the weather, and he sent the storm. I guess it depends on whether you're going by the book or the movies.