I don't think people *dislike* it per se. I think partly the problem is some people use the calendar solely for birthdays (which to me seems odd because birthdays are a social feature, while a calendar is more organisational) and others use the calendar as a calendar, but find it a bit limited.
If I see calender full of birthdays (especially on big forums) I don't even bother checking if there is some usefull event mentioned somewhere.