The metal construction is what prompted me to switch over to macbook pro's back in the day. The plastic dell laptops i used to use couldn't handle the abuse that it took during all of the travel i was doing at the time (cases kept cracking). I switched to a pro and was rewarded later with it surviving a 5 foot fall from a car rental counter. It bent part of the corner, but the screen was still in tact and it continued to work well enough to get me through the trip. I suspect the plastic alternative would have been toast.
Having kids today and seeing how rough they are with their toys, I'm not confident that a plastic laptop would survive them long.