The plastic case isn't going to crack unless you drop it. In the few years I've been around I have yet to see a laptop case break unless it was dropped. MBs are going to break unless the user does something to it.
Im not sure where you have "been around" but I never dropped my white crackbook and the inventible palmrest crack came along with small tiny little hairline cracks which developed all around the vents. I was always very careful with my computer as I was with my launch day white iPhone 3g which also developed a crack.
Go search, palmrest crack in Google and you will find PLENTY of people with the same cracks, as well as in the iPhone 3G discussion about their cracks.
But I guess...they all "dropped" theirs.
The plastic is not of good quality or at least there was a HUGE bad batch.