The benifit to on-line check-in is that if you have room "request" or if you really do need the pack n play, its a great way to let them know ahead of time and not just spring stuff on them when you get there.
You can make requests at any time - from the time you make your reservation, right up until you check in. Online checkin requests are usually limited to specific ones in a drop down box, and depending on the resort, may be completely unrelated to the request you want to make. Much better to call with the details of your request, if you didn't add them at the time you made your reservation.
Definitely this. In fact, I've had a CM tell me that with online check-in people, they try to get them into a room that will be ready at their arrival time before they even consider any of their room requests. Last time I did online check-in, I got the exact opposite of my room request because it was a room that was ready when arrived. I asked for on that met my request, and I didn't get a room until 4 hours later (which was fine by me).
Not a fan of on line check in. It doesn't guarantee your requests will be met, it doesn't even guarantee you will get the room type you booked (yep, had that lovely experience). And if you are at all fussy about what location your room is in, if you will request a different one if you don't like what they assign you, it will take longer than normal check in.
I usually get better room locations not doing on line check in.