I am sure that many guests have had a good experience with online check-in. I also know, from reading these boards, that there have been problems in some cases. We are less likely to hear from those with good experiences. I suspect that one of the reasons that you are asked for your time of arrival during the online process(even though you are reminded that check-in is after 4PM), is so that an attempt can be made to have your room ready when you arrive. Otherwise, why would they even bother to ask?