For an approach with a crosswind, it is preferable to fly crabbed into the wind. A crabbed approach requires less power and less fuel due to the decrease in drag and body forces. If you apply a forward slip (crosswind controls (low wing high rudder)) early, you increase the drag resulting in a higher power setting, pilot fatgue (holding the rudder for long periods), and is uncomfortable for the passengers.
I think what you are witnessing are heavies commencing their approaches in a crabbed condition, but, as they start their flare, they will transition to a forward slip (wing low, high rudder). Only the pros will wait until the last second to do this. Most folks I fly with will start at 200 feet or so with a slow rudder input to align the nose and a corresponding wing low to kill the rolling moment.
Aircraft are designed to withstand certain loads, check out
FAR Part § 25.485: Side load conditions -- FAA FARS, 14 CFR.
While on active duty, I was once tasked to find the max crosswind components for various aircraft. Boeing gave the max demonstrated and max theoretical crosswind components for their aircraft. Additionally, they also sent us the max crabbed landing components. These numbers were much lower than the other numbers.
The takeaway ... yes, they are designed for some loading, but not alot. If you don't kill the drift or land in too much crab, this happens ...
http://www.ntsb.gov/Events/2005/FedE..._animation.wmv
-Fatty