Well, it's flawed because it depends on people to support it and do the right thing. What, me cynical?
People lie in feedback, customers slap negs before contacting sellers to work it out, your feedback score doesn't necessarily reflect all of your transactions, etc.
It's too problematic and if it were mandatory (not that you're really suggesting that) it would be even worse.
I'm not saying it should go away, though -- it can be very useful when checking out another buyer/seller.
eBay should know it's bad advice to ask for feedback too. As you suggested above, sometimes you may get more than you bargained for. eBay's not the friendly, online swap meet that it apparently once was!