At what point did they get better? Like I said, mine's pushing three years old.
The plastic was supposed to get better with the Santa Rosa MacBooks (Late 2007), however that was stated before people had time to use the computers in the real world. Reports since then show that the issue still exists. Once Apple issued the support document stating that repairs should be free, people assumed they'd improve the plastic as much as they could to cut down on the repair bills.
The magnets in the MB are not strong enough to break plastic - if they were, they'd wipe the hard disk clean, and it would take brute force to open the lid.
I never said the magnetic forces were breaking the plastic, that would be stupid.
Due to the magnetic attraction between the lid and the palmrest of the computer, the "feet" are pushed into the palmrest with extra force during the last few centimetres of closure. It is very difficult to close the lid without either letting the feet hit the palmrest or without excessively babying the lid shut. Doing the latter usually results in one side of the lid closing before the other, which probably isn't good for the hinge.
The "feet" in the lid are capable of pressing down into the palmrest and causing cracks if it slams down hard enough, but I find it pretty much unbelievable that simply letting the magnets pull on an already closed lid will cause the plastic to break.
As the magnets are holding the lid shut, normal transportation of the computer (in any sort of case, bag or even just on its own) will result in the lid and palmrest magnets pulling at each other. Over time the movement of the magnets within the lid/palmrest can damage the plastic. If you want to see how much the magnets can move, try sliding the Apple Remote over the magnets (you should hear the magnet move).
Like I pointed out,
this has been a godsend. That, and closing the lid gently.
You shouldn't have to buy an additional product in order for a Notebook computer to perform such a basic function (i.e. closing).
You could shove a cushion in between the lid and the palmrest. That might solve the problem, but it shouldn't be necessary.
Your anecdotal evidence is somewhat tainted given that you've been using a protective product. If you didn't use that, then you might too be experiencing this issue.
I still stand by the fact that Apple recognises this issue. They are generally very happy to ignore issues like this when they don't feel they are at fault. In this case, they're willing to replace several parts on computers that are over three years old because they feel that the product simply isn't fit for purpose.
Sure if you keep the computer in a bubble it might not break, but you really shouldn't have to.
I know someone that picks his Black MacBook up with a cloth because he protects it so much, it developed cracks after a year.
