I have actually sometimes used online checkin at Marriott, because it's useful to find out what room you have been assigned (i.e., sometimes there are certain rooms or types of rooms that I either prefer or want to avoid, and the room number is often a good clue). It might even enable you to check in early, in some cases.

On the other hand, if it's a property that I already KNOW doesn't really make use of online checkin (based on past experience), I'm not going to try to use it.

Either way, the online chat feature can be very helpful. I would definitely use it to alert the hotel to late arrival. I also have used it (for example) to request a "late" checkout (such as 12 noon instead of 11 AM). It's good to have the "paper trail" (or rather, a "screenshot trail") as someone else mentioned.