My first Intel Mac was a 2GHz MB from 2006 and I was lucky to have AppleCare as body parts got replaced a few times, including every single piece of the case apart from the top.
I was desperate for either a MacBook Air or any new aluminium MacBook. I ordered one on the same day when the first unibodies came out, despite the lack of Firewire.
I liked my plastic MacBook, but it's just a fact of life that that sort of plastic is fragile and it breaks easily EVEN if there is no manufacturing fault or design mistake.
My only suggestion to the owners would be to get the cracked parts replaced and then get an aluminium one, unless the glossy screen is a problem.