The point of check-in is now different than it used to be, but it's very important. The new purpose is to make sure all of the non-frequent fliers aren't confused.
Think about it - there are occasional fliers out there that still don't know how to handle TSA. You've all seen them.. you know the people that aren't sure what to put in their quart-sized baggie or are unsure of whether to take off their shoes. If we eliminated check-in, these guys would have no idea of what to do! It would be chaos, so I say, let's keep it - it's easy for us anyhow and perhaps it helps a few people out there from getting too confused and somehow slowing down the security line more than they already do!