Are diamonds with great specs on paper always a good deal?
I get that strategy... But there's more to a diamond than stats. This is especially true for LGs, which can have so many defects that don't show up on reports (strain, striations, undertone, etc.).
Think of it this way - if you received a diamond that was 10% "less sparkly" than it was supposed to be, and you hadn't had a chance to compare it against other stones, how would you know? You wouldn't until your new fiancee was comparing it against her friends' rings - and by then, it's too late.