Louis Vuitton is a bad example. Some designer labels, such as Yves Saint Laurent and Balenciaga, use top quality materials, with impressively strong, small, close-set stitching, and impeccably tasteful design that does not compromise rugged construction, resulting in products that can last a surprisingly long time in near-mint condition with good care and periodic maintenance. Some of those bags really are worth the $1300 or more that they cost due to long-lived utility and artistic value (and you can often find them for 50% off or better in the resale market, which is worth trying IF AND ONLY IF you know how to spot a fraud). Small changes in manufacturing can make a big difference in quality of result... and knock offs tend to be significantly inferior to the brand-name item due to lots of changed details, often stemming from cutting corners. Just because a rip off "[came] off the same assembly line as the real ones" doesn't mean it was made the same way.
But Louis Vuitton? Not that great a bag. The LV design house is a miracle of marketing, which is not entirely surprising given the special place they occupy in apparel-related trademark history.
I'm not really a bag connoisseur. I just married one.