Except all of that can be erased pretty easily. One could rewrite the firmware on the CPU (Yes, people do that..). One could alter the DMI information to get rid of memory manufacturer dates and serial numbers. MAC addresses can't be used as a unique identifying device either. NIC manufacturers produce more than one card with any given MAC address. There simple isn't enough addresses to go around. There was a recent, very informative thread on NANOG about this very thing.
Even if the serial numbers were even remotely useful in tracking the computer's origins, it would track only that : It's origins. Computer hardware changes hands way too often with no form of tracking between private parties. How many people here on Slashdot have rescued a "dead" computer from an employer's trash bin and brought it back to life? How many retail stores do you know of that track every item sold down to its individual serial number? Maybe Best Buy does this, but 90% of your "Mom and Pop" computer stores aren't going to do this.
TL;DR version : Serial numbers on a computer are useless for tracking.
