Fergie has a point, but I feel he playing it down a bit to make that point. Different coaches have different philosophies so some will value a great WC performance more than others. I think the WC has biggest upside for young up and coming or relatively unknown players. Also, I think many of the purchasing decisions are made by football directors who are just as interested in buying the next hot thing for marketing reasons as much as for football reasons ("Selling shirts...etc").
At the end of the day...purchasing any player has some element of risk. A great club player in one country could flop in a different league or even at a different club in the same league. I think if you are a talented well-known player...then a poor WC performance does not hurt your career that much. If you are an average or sub-par club player and have a great WC, some coaches may think it was a flash in the pan and still may not take the risk, but it certainly does not hurt...you are now on the radar and the increased confidence could propel your game to the next level.
'Unknown' players may seem to get snapped up based on great WC performances, but in many cases, clubs already had an eye on them and if they did not, will then proceed to do a bit of a background check. However if you are unkown and crash at the WC..you will probably not attract any attention. So a good WC performance could only help...someone will always be willing to take the risk on you...or at the very least, they will all be watching to see what you do next